add new output

def cmd_output_add(self, args):
        '''add new output'''
        device = args[0]
        print("Adding output %s" % device)
        try:
            conn = mavutil.mavlink_connection(device, input=False, source_system=self.settings.source_system)
            conn.mav.srcComponent = self.settings.source_component
        except Exception:
            print("Failed to connect to %s" % device)
            return
        self.mpstate.mav_outputs.append(conn)
        try:
            mp_util.child_fd_list_add(conn.port.fileno())
        except Exception:
            pass