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