Make notes extensible

[knowtheory]

Talking with Jacob Fenton (@jsfenfen) about his JSK Fellowship.

He's building an interface to draw bounding boxes on top of page images.

Since we've already got a framework for displaying notes... it might be interesting to make the type of notes and note views swappable.

Being able to replace the NoteView initialization (or make it settable in the initialization options would be super handy.

[knowtheory]

Note, this is entirely exploratory/experimental/random