Add support for passing a method to filters generated through AutoFilter

[RouganStriker]

Allow AutoFilter to accept a method param and pass it to the generated filters.

The same behaviour for the following

class MyFilter(FilterSet):
    title__contains = CharFilter(method='custom_pk_filter')
    title__endswith = CharFilter(method='custom_pk_filter')

    def custom_pk_filter(self, qs, field, value):
        pass

Can now be done as

class MyFilter(FilterSet):
    title = AutoFilter(lookups=['contains', 'endswith'], method='custom_pk_filter')

    def custom_pk_filter(self, qs, field, value):
        pass

[rpkilby]

Thanks for submitting this. For now, this is blocked by carltongibson/django-filter#1150, which would make the field_name/lookup_expr workaround unnecessary.