Add the "Extending Phabricator" config group.

Summary: Refs T2255 and takes care of the "EXTENDING PHABRICATOR" group thereof.

Test Plan: Looked at each of the new options.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4325