Add specialcase area to remarkup when on a dark background.

Summary: Remarkup dark wrapper for Pholio/Remarkup, reusable

Test Plan: Tested a blockquote, list, and code segment

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2640

Differential Revision: https://secure.phabricator.com/D5196