Parse inline comments via remarkup engine

Summary: Inline comments weren't haven't remarkup applied. Fixes T3137

Test Plan: Added some inline comments, checked that they had remarkup applied. Also checked in the real time preview.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3137

Differential Revision: https://secure.phabricator.com/D6024