Add validation for config settings of type regex

Summary: Also fixes insufficiently-escaped regex examples

Test Plan: Made several changes to http://local.phacility.com/config/edit/syntax.filemap/ and observed validation failures on malformed regexes, and success on well-formed regexes.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12532

Differential Revision: https://secure.phabricator.com/D17684