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.

cov-core==1.15.0
coverage==4.0.3
invoke==0.21.0
mock==1.0.1
pytest==2.8.7
pytest-cov==2.2.1
pytest-mock==0.10.1