Log Message:
Import cmark-gfm-0.29.0.gfm.0

cmark-gfm is an extended version of the C reference implementation of
CommonMark, a rationalized version of Markdown syntax with a spec.
This package adds GitHub Flavored Markdown extensions to the upstream
implementation (https://github.com/jgm/cmark), as defined in the spec
(https://github.github.com/gfm/).