Function taking a parsed template tag
    to a MacroArgNode.

def do_macro_arg(parser, token):
    """ Function taking a parsed template tag
    to a MacroArgNode.
    """
    # macro_arg takes no arguments, so we don't
    # need to split the token/do validation.
    nodelist = parser.parse(('endmacro_arg',))
    parser.delete_first_token()
    # simply save the contents to a MacroArgNode.
    return MacroArgNode(nodelist)