Added a new markup rule for deleted text
Summary: Deleted text (aka strikethrough) which produces <del>deleted text</del>
Test Plan: Created unit test case in the tests directory. Ran unit tests and they all pass.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, epriestley
Differential Revision: https://secure.phabricator.com/D1941