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