Add `MultipleChoice` field.

[lovelydinosaur]

Display as a <select multiple>

[jhtimmins]

Seems like this could either subclass Choice or inherit from Field directly. Do you have a preference? Also, I'm thinking it should include "min selected" and "max selected" as options. Anything else?

I'm happy to work on other fields/issues first if they're higher priority.

[lovelydinosaur]

I'd suggest:

• Subclass Field.
• Don't include min selected / max selected, at least for the initial pass. That can come later.

We'll want to think carefully about null and empty behaviors, eg:

• We won't want to render an initial blank <choice> in the multiple case.
• Forms with no values selected won't submit any value at all. For consistency we probably want MultipleChoice(default=[]) to be the typical usage - similar to BooleanField(default=False) being the correct way to have a checkbox that acts as False if it does not submit a value.