Diffusion - re-jigger how README files get rendered
Summary: be more aggressive about assuming plain-text, use remarkup for no extension, .remarkup, and .md, and last but not least use rainbow for .rainbow. Fixes T5818.
Test Plan: my README rendered just fine post these changes
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: asherkin, epriestley, Korvin
Maniphest Tasks: T5818
Differential Revision: https://secure.phabricator.com/D10340