This uses the [cmarkgfm](https://pypi.org/project/cmarkgfm/) package to render the "gfm" variant of Markdown. It also preferentially uses cmarkgfm for CommonMark as it is CommonMark compliant (and fast).

Notes:
* The cmarkgfm library is currently an *extra* dependency (``readme_renderer[gfm]``) mostly because I don't yet have wheels for cmarkgfm.
* I added a ton of fixtures for gfm based on GitHub's [own spec](https://github.github.com/gfm/). They had to be tweaked a little because bleach changes the output very slightly (such as adding `rel="nofollow"`).