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.

--exclude=build
--exclude=dist
--exclude=docs/_cache
--exclude=docs/_counter
--exclude=bower_components
--exclude=node_modules
--exclude=docs/raw
--exclude=*.egg-info
--exclude=__pycache__
--exclude=__tmpfs__