Make remarkup rules runtime-pluggable in a reasonable way

Summary:
Gets rid of some old Differential-specific nonsense and replaces it with general runtime-pluggable Remarkup rules.

Facebook: This removes two options which may be in use. Have any classes being added via config here just subclass the new abstract bases instead. This should take 5 seconds to fix. You can adjust order by overriding `getPriority()` on the rules, if necessary.

Test Plan: See comments.

Reviewers: btrahan

Reviewed By: btrahan

CC: FacebookPOC, andrewjcg, aran

Differential Revision: https://secure.phabricator.com/D7393