Interpret "## text"-style Markdown headers
Summary: Ref T5102. This conflicts with # lists, but we can resolve the ambiguity with a reasonable heuristic.
Test Plan: Added and executed unit tests.
Reviewers: asherkin, btrahan, chad
Reviewed By: chad
Subscribers: epriestley
Maniphest Tasks: T5102
Differential Revision: https://secure.phabricator.com/D10045