Allow lint messages to be rendered as Remarkup

Summary: Some linter messages, such as those produced by ArcanistPHPCompatibilityXHPASTLinterRule, contain backticks but are currently rendered as Remarkup literals. I think that it is generally desirable to allow lint messages to be rendered as Remarkup, although we should ideally have a way to render Remarkup for use on the command line (I actually think that this already exists, but I don't think that arc lint does this when rendering linter messages).

Test Plan: Resubmitted D14481 to my dev install and saw Remarkuped lint messages.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14485