R1:460a462164c0

Use "~~del~~" remarkup style in Phabricator

Summary: Add @20after4's new rule to the Phabricator engine.

Test Plan: Wrote some ~~deleted~~ text.

Reviewers: 20after4, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1972