Add a Diffusion repository remarkup rule
Summary: Currently we markup rXabcd, but not rX on its own. Mark these up as repository object names.
Test Plan: Typed rPOEMS, rPOEMS1, rPOEMS139893189, etc.
Reviewers: btrahan, dctrwatson
Reviewed By: btrahan
CC: aran, poop
Differential Revision: https://secure.phabricator.com/D7859