Replace gfm with commonmark

[gordonbrander]

Fixes #8.

GFM doesn't ignore inline HTML when rendering, and will produce bugs,
such as re-hrefing the href in a link tag.

GFM has essentially been abandoned. This PR replaces it with commonmark,
which implements Common Markdown (https://commonmark.org/).

Pros:

• Commonmark is maintained by readthedocs, so is a good bet in terms of
  code durability.
• Edge-cases in CommonMark are well-specified.
• The library generates an AST, which we could use for enhancements
  later.

Cons:

• No auto-linking of bare URLs. You need to wrap URLs in
• No br for soft linebreaks. you will need to add two spaces at the end
  of the line to create a BR.