Markdown provider: Pass path & file info

[abovedave]

Would be handy to pass info about the file to the JSON for use in the front end. For example, path and filename.

/posts/setup/testing.md

{
        "original": "---\ndate: 2016-04-17\ntitle: Testing\nhandle: pages\nauthor: The DADI Team\n---\n\nhow to create a new page...",
        "attributes": {
          "date": "2016-04-17T00:00:00.000Z",
          "title": "Testing",
          "author": "The DADI Team"
        },
        "path": [
          "posts",
          "setup"
        ],
        "filename": "testing",
        "contentText": "how to create a new page...",
        "contentHtml": "<p>how to create a new page...</p>\n"
}

[abovedave]

Ok, I've noticed we don't actually support a nested directory structure, so I will add that in. [ed: now done]

From my last commit, I've added some additional values to the attributes object:

"attributes": {
  "date": "2016-05-10T00:00:00.000Z",
  "title": "Test folder reader",
  "author": "The DADI Team",
  "_loc": "workspace/posts/folder/filename.md",
  "_ext": ".md",
  "_id": "filename",
  "_path": [
    "workspace",
    "posts",
    "folder"
  ]
}

I noticed if you added a non-text file into the file directory, it caused an error (e.g., an image). I've added support for an extension parameter which will only look for files with the given extension (the default being .md files). The datasource schema looks like:

{
  "datasource": {
    "key": "posts",
    "name": "Blog posts as markdown files.",
    "source": {
      "type": "markdown",
      "path": "./workspace/posts",
      "extension": "txt"
    }
}