Extra widgets

Hi Stephen, I think would be good if you can set extra widgets for extra or default fields.
for example:

FORMS_BUILDER_EXTRA_FIELDS = (
    (100, 'my_module.fields.MyCustomField', 'My field'),
    (101, 'my_module.fields.MyCustomField', 'My field', 'my_module.widgets.MyCustomWidget'),
    (102, 'my_module.fields.MyCustomField', 'My field', ('email', 'django.forms.widgets.TextInput')),  # html5
    (103, 'my_module.fields.MyCustomField', 'My field'),
)

FORMS_BUILDER_EXTRA_WIDGETS = (
    (103, ('date', 'django.forms.widgets.DateInput')),
    (10, 'django.forms.widgets.DateInput'), 
)