Add enum option type.

Summary: Might not be the cleanest way to do this, but seems to work.

Test Plan:
- Saved an option which used the new enum type.
- Changed it.
- Saw it show up on the list view.

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4572