formatting: Add a `hoedown` formatter. The `hoedown` library is not yet in the dependency list however since I have to figure out its availability on most platforms.

cov-core==1.15.0
coverage==4.0.3
invoke==0.12.2
mock==1.0.1
pytest==2.8.7
pytest-cov==2.2.1
pytest-mock==0.10.1