A nicer way to do extra CSS classes in form_field macro

I think this is a lot tidier, popping thing from kwargs before sending them to the field (which I had in the previous commit), is a bit messy.

This introduce a new optional second argument to the form_field macro, which has to be a dict.

 In this dict we put stuff that is specific to the macro, extra stuff for certain fields basically, however we don't want to send that to the field() method... which is what we still use the kwargs for.