[Feature] Create API for parsing to AST and NSAttributedString (#132)

* Create Node wrapper

* Categorize nodes into classes

* Add debug descriptions

* Define Visitor

* Add simple test for demonstration purposes

* Move nodes into separate files

* Move visitors into separate files

* Tidy up

* Expose Visitor protocol

* Add accept method to Document

* Update various nodes

* WIP: AttributedStringVisitor

* Fill out remaining visit methods

* Move Styler protocol into its own file

* Append blank line only for nodes with successors

* Add prefixes to list items

* Style list prefixes

* Rename isLast

* Adjust accept signature

* Adjust Image and Link nodes

* Pass url to styler methods

* Provide API to render to an attributed string via the AST

* Fix Link & Image properties

* Pass fenceInfo to styler method

* Slight refactoring of nodes

* Improve DebugVisitor to print tree depth

* Tidy up

* More tidy up

* Add some descriptions

* Tidy up

* Add documentation

* Make nodes public

* Tidy up

* More documentation

* Add visitor test

* Fix compile error for Xcode 10.1

* Remove unnecessary line breaks from blocks

* Test AttributedStringVisitor

* Make lazy var private(set)

* Clean up TODO

* Make all cmarkNode reference immutable

* Don’t fatalError on list type

* Don’t fatalError when wrapping child nodes

* Tidy up

* Don’t fatalError when visiting children

* Tidy up doc comments

* Improve documentation

* Refactor Node initialization

* Expose wrap() method

* Tidy up

* Fix warnings

* Lazily compute children

* Fix typo

Source/AST/Nodes/BaseNode.swift

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+//
+//  BaseNode.swift
+//  Down
+//
+//  Created by John Nguyen on 21.04.19.
+//
+//
+
+import Foundation
+import libcmark
+
+public class BaseNode: Node {
+
+    public let cmarkNode: CMarkNode
+
+    public private(set) lazy var children: [Node] = {
+        var result: [Node] = []
+        var child = cmark_node_first_child(cmarkNode)
+
+        while let raw = child {
+
+            guard let node = raw.wrap() else {
+                assertionFailure("Couldn't wrap node of type: \(raw.type)")
+                continue
+            }
+
+            result.append(node)
+            child = cmark_node_next(child)
+        }
+
+        return result
+    }()
+
+    init(cmarkNode: CMarkNode) {
+        self.cmarkNode = cmarkNode
+    }
+}

Source/AST/Nodes/BlockQuote.swift

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+//
+//  BlockQuote.swift
+//  Down
+//
+//  Created by John Nguyen on 09.04.19.
+//
+
+import Foundation
+import libcmark
+
+public class BlockQuote: BaseNode {}
+
+
+// MARK: - Debug
+
+extension BlockQuote: CustomDebugStringConvertible {
+
+    public var debugDescription: String {
+        return "Block Quote"
+    }
+}

Source/AST/Nodes/Code.swift

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+//
+//  Code.swift
+//  Down
+//
+//  Created by John Nguyen on 09.04.19.
+//
+
+import Foundation
+import libcmark
+
+public class Code: BaseNode {
+
+    /// The code content, if present.
+    public private(set) lazy var literal: String? = cmarkNode.literal
+}
+
+
+// MARK: - Debug
+
+extension Code: CustomDebugStringConvertible {
+
+    public var debugDescription: String {
+        return "Code - \(literal ?? "nil")"
+    }
+}

Source/AST/Nodes/CodeBlock.swift

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+//
+//  CodeBlock.swift
+//  Down
+//
+//  Created by John Nguyen on 09.04.19.
+//
+
+import Foundation
+import libcmark
+
+public class CodeBlock: BaseNode {
+
+    /// The code content, if present.
+    public private(set) lazy var literal: String? = cmarkNode.literal
+
+    /// The fence info is an optional string that trails the opening sequence of backticks.
+    /// It can be used to provide some contextual information about the block, such as
+    /// the name of a programming language.
+    ///
+    /// For example:
+    /// ```
+    /// '''<fence info>
+    /// <literal>
+    /// '''
+    /// ```
+    ///
+    public private(set) lazy var fenceInfo: String? = cmarkNode.fenceInfo
+}
+
+
+// MARK: - Debug
+
+extension CodeBlock: CustomDebugStringConvertible {
+
+    public var debugDescription: String {
+        return "Code Block - \(literal ?? "nil"), fenceInfo: \(fenceInfo ?? "nil")"
+    }
+}

Source/AST/Nodes/CustomBlock.swift

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+//
+//  CustomBlock.swift
+//  Down
+//
+//  Created by John Nguyen on 09.04.19.
+//
+
+import Foundation
+import libcmark
+
+public class CustomBlock: BaseNode {
+
+    /// The custom content, if present.
+    public private(set) lazy var literal: String? = cmarkNode.literal
+}
+
+
+// MARK: - Debug
+
+extension CustomBlock: CustomDebugStringConvertible {
+
+    public var debugDescription: String {
+        return "Custom Block - \(literal ?? "nil")"
+    }
+}

Source/AST/Nodes/CustomInline.swift

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+//
+//  CustomInline.swift
+//  Down
+//
+//  Created by John Nguyen on 09.04.19.
+//
+
+import Foundation
+import libcmark
+
+public class CustomInline: BaseNode {
+
+    /// The custom content, if present.
+    public private(set) lazy var literal: String? = cmarkNode.literal
+}
+
+
+// MARK: - Debug
+
+extension CustomInline: CustomDebugStringConvertible {
+
+    public var debugDescription: String {
+        return "Custom Inline - \(literal ?? "nil")"
+    }
+}

Source/AST/Nodes/Document.swift

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+//
+//  Document.swift
+//  Down
+//
+//  Created by John Nguyen on 09.04.19.
+//
+
+import Foundation
+import libcmark
+
+public class Document: BaseNode {
+
+    deinit {
+        // Frees the node and all its children.
+        cmark_node_free(cmarkNode)
+    }
+
+    /// Accepts the given visitor and return its result.
+    public func accept<T: Visitor>(_ visitor: T) -> T.Result {
+        return visitor.visit(document: self)
+    }
+}
+
+
+// MARK: - Debug
+
+extension Document: CustomDebugStringConvertible {
+
+    public var debugDescription: String {
+        return "Document"
+    }
+}

Source/AST/Nodes/Emphasis.swift

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+//
+//  Emphasis.swift
+//  Down
+//
+//  Created by John Nguyen on 09.04.19.
+//
+
+import Foundation
+import libcmark
+
+public class Emphasis: BaseNode {}
+
+
+// MARK: - Debug
+
+extension Emphasis: CustomDebugStringConvertible {
+
+    public var debugDescription: String {
+        return "Emphasis"
+    }
+}

Source/AST/Nodes/Heading.swift

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+//
+//  Heading.swift
+//  Down
+//
+//  Created by John Nguyen on 09.04.19.
+//
+
+import Foundation
+import libcmark
+
+public class Heading: BaseNode {
+
+    /// The level of the heading, a value between 1 and 6.
+    public private(set) lazy var headingLevel: Int = cmarkNode.headingLevel
+}
+
+
+// MARK: - Debug
+
+extension Heading: CustomDebugStringConvertible {
+
+    public var debugDescription: String {
+        return "Heading - L\(headingLevel)"
+    }
+}

Source/AST/Nodes/HtmlBlock.swift

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+//
+//  HtmlBlock.swift
+//  Down
+//
+//  Created by John Nguyen on 09.04.19.
+//
+
+import Foundation
+import libcmark
+
+public class HtmlBlock: BaseNode {
+
+    /// The html content, if present.
+    public private(set) lazy var literal: String? = cmarkNode.literal
+}
+
+
+// MARK: - Debug
+
+extension HtmlBlock: CustomDebugStringConvertible {
+
+    public var debugDescription: String {
+        return "Html Block - \(literal ?? "nil")"
+    }
+}

Source/AST/Nodes/HtmlInline.swift

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+//
+//  HtmlInline.swift
+//  Down
+//
+//  Created by John Nguyen on 09.04.19.
+//
+
+import Foundation
+import libcmark
+
+public class HtmlInline: BaseNode {
+
+    /// The html tag, if present.
+    public private(set) lazy var literal: String? = cmarkNode.literal
+}
+
+
+// MARK: - Debug
+
+extension HtmlInline: CustomDebugStringConvertible {
+
+    public var debugDescription: String {
+        return "Html Inline - \(literal ?? "nil")"
+    }
+}

Source/AST/Nodes/Image.swift

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+//
+//  Image.swift
+//  Down
+//
+//  Created by John Nguyen on 09.04.19.
+//
+
+import Foundation
+import libcmark
+
+public class Image: BaseNode {
+
+    /// The title of the image, if present.
+    ///
+    /// In the example below, the first line is a reference link, with the reference at the
+    /// bottom. `<text>` is literal text belonging to children nodes. The title occurs
+    /// after the url and is optional.
+    ///
+    /// ```
+    /// ![<text>][<id>]
+    /// ...
+    /// [<id>]: <url> "<title>"
+    /// ```
+    ///
+    public private(set) lazy var title: String? = cmarkNode.title
+
+    /// The url of the image, if present.
+    ///
+    /// For example:
+    ///
+    /// ```
+    /// ![<text>](<url>)
+    /// ```
+    ///
+    public private(set) lazy var url: String? = cmarkNode.url
+}
+
+
+// MARK: - Debug
+
+extension Image: CustomDebugStringConvertible {
+
+    public var debugDescription: String {
+        return "Image - title: \(title ?? "nil"), url: \(url ?? "nil"))"
+    }
+}

Source/AST/Nodes/Item.swift

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+//
+//  Item.swift
+//  Down
+//
+//  Created by John Nguyen on 09.04.19.
+//
+
+import Foundation
+import libcmark
+
+public class Item: BaseNode {}
+
+
+// MARK: - Debug
+
+extension Item: CustomDebugStringConvertible {
+
+    public var debugDescription: String {
+        return "Item"
+    }
+}