Allow item-editor to take over configurable elements.

Also added class names in addition to ids. This was my first cut - the
version used in perseus 360 - I'm thinking it would probably make more
sense to pass in elements rather than selectors, but I want to ship
this.

Test Plan:
Integrated in perseus one I'm able to edit questions, hints, and answers
and see the modifications appear.

Auditors: alex
CC: alpert