Process Markdown-style headers in Remarkup

Summary:
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.

Test Plan:
Added a bunch of unit tests.

{F42635}

Reviewers: btrahan, jevripio

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2849

Differential Revision: https://secure.phabricator.com/D5854