This was already dealing with a rewrite due to the spec change on CommonMark, so
I took the opportunity to modernize the code and clean things up as well.
* Updates to support CommonMark==0.7.3
* Reworks pattern for handling CommonMark nodes, mimic docutils visit/depart
pattern
* Drops SectionHandler class, as it was only used in one place. Merge into the
parser class, where it is used
* Drops requirements file for setup.py config, as this is a package requirements
is superfluous
* Adds a number of more detailed test cases
* Adds test runner for 3.5, 3.6
* Updates strictness on prospector, cleans up linting errors and docstring
problems
