Remarkup - add underline rule

Summary: we need this for legalese. Ref T3116

Test Plan: made a legalpad document with underlines. also re-gened docs and noted underlines worked

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D7996