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