Process Markdown-style headers in Remarkup
Markdown supports a header style where headers are underlined with = or -. Support this in Remarkup as well.
I made this the text target rendering style too, since I think it looks a little nicer. Ref T2849.
Added a bunch of unit tests.
Reviewers: btrahan, jevripio
Reviewed By: btrahan
Maniphest Tasks: T2849
Differential Revision: https://secure.phabricator.com/D5854