Get the config data for a charm.

        @param charm_id The charm's id.
        @param channel Optional channel name.

def config(self, charm_id, channel=None):
        '''Get the config data for a charm.

        @param charm_id The charm's id.
        @param channel Optional channel name.
        '''
        url = '{}/{}/meta/charm-config'.format(self.url, _get_path(charm_id))
        data = self._get(_add_channel(url, channel))
        return data.json()