Add support for GFM features

[zrosenbauer]

Describe the problem you'd like to have solved

Currently the entire GFM spec is not supported, we need to support all features (i.e. footnotes).

Features

• Codeblocks
• Emojis
• Images wrapped in Link (anchor)
• Alerts
• Footnotes
• Checklists
• Comments

Describe the ideal solution

Add support for Footnotes and all other features (will need to double check).

Alternatives and current work-arounds

You can hard code certain features using txt => txt.text()

Additional context

• https://github.github.com/gfm/