Add form_callback argument for formset_factory

[Maxim Danilov]

it should be same for formset_factory like in modelform_factory

modelform_factory(model, form=ModelForm, fields=None, exclude=None, formfield_callback=None,  widgets=None)

formfield_callback for every field in form_factory,
form_callback for every form in formset_factory.

[Tim Graham]

For model forms, formfield_callback is a callable that takes a model field and returns a form field. This doesn't seem relevant for non-model forms. Please explain the use case a bit more.