Add support for commonmark extensions

[ollieread]

Add supports for the default commonmark extensions.

• Autolink
• Github Flavoured Markdown
• Footnote
• Table of Contents

[ollieread]

The Autolink extension can be added easily because it requires no new renders, just a test to make sure it's working.

Subtask: #5

[ollieread]

The Github Flavoured Markdown extension requires a custom inline renderer.

Subtask: #6

[ollieread]

The Footnote extension requires 4 custom renderers, 2 block and 2 inline.

Subtask: #7

[ollieread]

The Table of Contents extension will require some custom work to use blade files to generate the TOC markup. Sadly the whole extension is marked as final.

• Create custom table of contents extension that uses blade files
• Add unit tests