Response fields filtering/masking#79

Hi, I'd like to know if there is an easy way to modify the patch to mask the response fields by a query parameter (like /api/users?fields=email,name), I've tried the restplus approach with the X-Fields but that no longer works with the patch. I thought the way to go would be to use a schema factory as shown in then marshmallow docs: http://webargs.readthedocs.io/en/latest/advanced.html. But I don't understand the code base enough to intervene.