R1:e5ecd784eca7

Tweak Maniphest custom fields

Summary:
- Fix a bug where 'caption' didn't do anything.
- Provide an abstract base implementation for extensions.
- Add some documentation.
- Expose aux fields via conduit.

Test Plan: Added some fields like "Dinosaur", "Kilograms" and "derp" on my local
install. Read documentation.
Reviewed By: jungejason
Reviewers: hunterbridges, jungejason, tuomaspelkonen, aran
CC: aran, philc, jungejason
Differential Revision: 785