Add field parameter functions

[lafrech]

Add a layer above field2propertyto allow for more customization of the result.

Generalizes the multiplemechanism to deal with not only List.

There might be room for factorization with attribute functions by reworking FieldConverterMixin but field2property is also called in fields2jsonschema and I don't want to break something there.

Closes #500.

Still needs tests, doc, and I'd like to add an example of documenting webargs'DelimitedList.

[lafrech]

@Bangertm @sloria, I intend to merge this soon. The other v6 PRs should be easy so we're close to v6.

No pressure for a review. If you want to check this, I can wait. Otherwise, no problem.