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).

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/