Support content-type callables

[gabisurita]

http://cornice.readthedocs.io/en/latest/validation.html?highlight=content_type#id4

[glasserc]

How could this work? Generate a fake request, call the callable, and use what it returns?

[gabisurita]

That's an approach, or I think we could also leave an empty field (accept everything). It's just important that we make sure that a callable won't break the generator.