[facet] add extra_kwargs instance property on CWFacetedSearch

1      def __init__(self, query=None,
2                   filters={},
3                   doc_types=None,
4                   index=None,
5                   parents_for=None,
6 -                 children_for=None):
7 +                 children_for=None,
8 +                 **kwargs):
9          if index:
10              self.index = index
11          if doc_types:
12              self.doc_types = doc_types
13          self.parents_for = parents_for
14          self.children_for = children_for
15 +        self.extra_kwargs = kwargs
16          super(CWFacetedSearch, self).__init__(query, filters)
17 
18      def query(self, search, query):
19          if query:
20              return compose_search(search,