Enhance flexibiliy of SelectFiles output list coercion

After running into this use-case with my very own scripts (and thinking hard enough to come up with what I think is an elegant implementation), I've decided that allowing field-level control over output list coercion makes sense. Now `SelectFiles.inputs.force_lists` takes either a boolean that applies to all output fields or a list of output field names that should be a list. This pull request is limited to that narrow change, which is totally backwards compatible.