Add remarkup.process conduit method

Summary: This exposes a few remarkup engines over conduit.

Test Plan:
Local conduit console, and playing with

'cat example.json | arc call-conduit remarkup.process'

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1551