extra_views.formsets: Add 'widgets' support to BaseFormSetMixin

Add the [new-in-Django-1.6 `widgets` field][1].  Use like:

    class ItemInline(InlineFormSet):
        model = Item
        widgets = {
            'your_date_field': forms.DateInput(attrs={'class': 'datepicker'}),
        }

Partially addresses #83.

[1]: https://docs.djangoproject.com/en/1.7/ref/forms/models/#django.forms.models.modelformset_factory