Generalise isinstance dict check #185
Description
I have a 'dict' like class that actually implements the Python collections.MutableMapping interface. However, pystache will not render templates correctly with this object as it is not considered a dict by the check in _get_value (https://github.com/defunkt/pystache/blob/master/pystache/context.py#L46).
At present I have resorted to also inheriting from dict in my class, but it would be good if this check could be improved (perhaps also test for Mapping interface conformity).