Implemented a <hr /> remarkup rule to Phabricator

Summary: Implemented a <hr /> remarkup rule to Phabricator

Test Plan: {F36306}

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2744

Differential Revision: https://secure.phabricator.com/D5370