formatters: Remove Hoedown formatter, split it off to a pluging. Its availability was spotty depending on the platform (because `misaka` doesn't have binaries available for even Windows, for instance). So it was split off to a plugin that the user can install if needed.

[pytest]
norecursedirs = .* _darcs CVS .svn .git .hg *.egg-info dist build bower_components node_modules venv tmp* _cache {args}