Add CommonMark markdown rendering option

[bakerkretzmar]

Adds the option to use league/commonmark to parse and render markdown. This can be enabled by setting the commonmark config to any value. To configure the CommonMark environment or add extensions, the commonmark config can be set to an array with config and extensions keys:

// config.php

return [
    'commonmark' => [
        'config' => ['enable_em' => false],
        'extensions' => [new AttributesExtension],
    ],
];

The default configuration includes 4 extensions: attributes, smart punctuation, strikethrough, and tables. Extensions can easily be added or removed by overriding the commonmark.extensions config key.

Two other things:

• I'm not sure MarkdownParserContract is strictly necessary, and I wouldn't mind getting rid of it if we can, I just needed something to bind to.
• Changing MarkdownParser to take a MarkdownParserContract in its constructor instead of a JigsawMarkdownParser is not technically a breaking change, right? We're widening that type, so all possible existing scenarios there will keep working, and the $parser property is not typed, so it could technically contain anything right now, which is also still the case. If someone's reaching into the underlying parser to do anything funky that would almost definitely break if they enabled this new parser, but it's optional so I think that's fine.