Acceptable formats (examples) for custom answer type.

Summary:
If all of the widgets inside a custom answer type have the same examples,
will show those examples.

Test Plan:
Added multiple expression or input-number widgets to custom answer
on test.html, instantly saw Acceptable formats being populated.
Did the same on local copy, then previewed with same results.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2749