Don't highlight whole text in () as URL

Summary:
Users are confused by it.

Also allow highlighting of [http://...] and such.

Test Plan:
(http://example.com fixes this.)
[http://example.com]

Reviewers: epriestley

Reviewed By: epriestley

CC: fzamore, aran, Korvin

Differential Revision: https://secure.phabricator.com/D4362