Add a custom question type

[wyattis]

The custom question should be able to access data in Trellis and return datum that can be inserted into the database. It should be possible to do one-time random sampling when the question comes up and display questions in a customizable way.

Custom question display fits well with #373.

One implementation could involve a question lifecycle with some kind of restricted query access to the database. Perhaps using the following lifecycle events:

onInit (api): Runs before the question is visited for the first time
onVisible (api): Runs every time this question is visible on the page
onLeave (api): Runs every time a user navigates away from the page with this question

The api object would need methods for querying data and storing datum.