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