Environment class should implement __contains__

[christarnowski]

Since webassets.Environment already implements magic methods such as __getitem__, it would be nice if the class provided __contains__ as well:

def __contains__(self, name):
    return name in self._named_bundles

This small feature comes in handy when iron-proofing against typos; it's also much more elegant than try...except KeyError: block.

Thanks!