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