Hi, Following the "it's Easier to Ask Forgiveness than Permission" principle, is it possible to modify 2 lines in [pystache.context:_get_value](https://github.com/defunkt/pystache/blob/master/pystache/context.py#L51) ? ``` python if isinstance(context, dict): try: return context[key] except KeyError: pass ``` This would make it possible to pass a `collections.defaultdict` to `pystache.render`, which does not work currently: ``` python pystache.render("Hello {{}} !", defaultdict(lambda: "Anonymous visitor")) ```