Migrate to a maintained markdown processor

[mrueg]

https://github.com/russross/blackfriday/tree/v2 was 3 years ago, it might make sense to move to a different markdown processor.

This seems to be a maintained fork of blackfriday: https://github.com/gomarkdown/markdown/

[corhere]

github.com/gomarkdown/markdown is not a drop-in replacement (ast.Node is an interface rather than a struct) so other markdown parsers would be well worth considering, too. https://github.com/yuin/goldmark looks like a promising option, and is CommonMark compliant!