Add option to randomize options in multiple-choice templates

[mitcho]

Good feature request by @VerbingNouns: for multiple-choice templates, it would be nice to have an option to have the Lister randomize the answer choices.

Technically, this would mean an option to shuffle the fields within an individual item.

A general case implementation would be nontrivial:

• How does this affect latin square-ing and balancing?
• Each logical "choice" could correspond to one field, but it could also correspond to multiple fields, which would have to be shuffled as groups.
• There are also fields that, depending on the template, shouldn't be shuffled. For example, contexts.

But there's definitely an important use case here.