Add getGroup to ConfigOptions

Summary: Adds core and apps grouping to configuration options, makes it somewhat easier to browse config options.

Test Plan: Set each option, review list. Breakdown is nearly 50/50 apps/core.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11722