Add markup syntax for a literal block
Summary:
Lines starting with the '|' character will undergo no further
markup processing.
Test Plan:
Tested in sandbox. Verified that other remarkup on lines prefixed
with '|' did not get processed.
Reviewers: epriestley, jungejason
Reviewed By: epriestley
CC: aran, andrewjcg, epriestley
Differential Revision: https://secure.phabricator.com/D1293