Diffusion / MetaMTA options

Summary: Implement Diffusion MetaMTA options. Also make the fake '{{config.option}}' rule work, and use Remarkup to render summaries as well as descriptions.

Test Plan: Looked at Diffusion rules, edited some, looked at setup issues, verified '{{config.option}}' linked to the right option.

Reviewers: codeblock, btrahan

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4466