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
Parse most `[name](href)` Markdown-style links in Remarkup

Summary: Markdown has `[name](href)`-style links which we currently don't parse because they can occur naturally in discussions of source code (instead, we support Mediawiki-style links). Apply a few heuristics to reject likely accidental matches and support reasonable forms of this style. Ref T2849.

Test Plan: Added unit tests.

Reviewers: btrahan, jevripio

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2849

Differential Revision: https://secure.phabricator.com/D5857