docs: def filter_options(cls, kwargs, keys):
          """
          Make optional kwargs valid and optimized for each template engines.
  
          :param kwargs: keyword arguements to process
          :param keys: optional argument names
  
          >>> Engine.filter_options(dict(aaa=1, bbb=2), ("aaa", ))
          {'aaa': 1}
          >>> Engine.filter_options(dict(bbb=2), ("aaa", ))
          {}
          """
          return dict((k, v) for k, v in filter_kwargs(keys, kwargs))