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