Create a new consumer group associated with a stream.
        name: name of the stream.
        groupname: name of the consumer group.
        id: ID of the last item in the stream to consider already delivered.

def xgroup_create(self, name, groupname, id='$', mkstream=False):
        """
        Create a new consumer group associated with a stream.
        name: name of the stream.
        groupname: name of the consumer group.
        id: ID of the last item in the stream to consider already delivered.
        """
        pieces = ['XGROUP CREATE', name, groupname, id]
        if mkstream:
            pieces.append(Token.get_token('MKSTREAM'))
        return self.execute_command(*pieces)