Enable use of remarkup literal block

Summary:
D1293 adds support for a literal block in remarkup. This diff enables
it in phabricator with a few basic rules (for line breaks, escaping HTML,
and linkifying URLs).

Test Plan: Tested in sandbox

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1297