docs: def command(self, command, token, team_id=None, methods=['GET'], **kwargs):
          """A decorator used to register a command.
          Example::
  
              @slack.command('your_command', token='your_token',
                             team_id='your_team_id', methods=['POST'])
              def your_method(**kwargs):
                  text = kwargs.get('text')
                  return slack.response(text)
  
          :param command: the command to register
          :param token: your command token provided by slack
          :param team_id: optional. your team_id provided by slack.
                          You can also specify the "TEAM_ID" in app
                          configuration file for one-team project
          :param methods: optional. HTTP methods which are accepted to
                          execute the command
          :param kwargs: optional. the optional arguments which will be passed
                         to your register method
          """
          if team_id is None:
              team_id = self.team_id
          if team_id is None:
              raise RuntimeError('TEAM_ID is not found in your configuration!')
  
          def deco(func):
              self._commands[(team_id, command)] = (func, token, methods, kwargs)
              return func
          return deco