Add a command function to the registry.

        :param func: command function.
        :param name: default name of func.
        :param doc: description of the func.default docstring of func.

def add_command(self, func, name=None, doc=None):
        """
        Add a command function to the registry.

        :param func: command function.
        :param name: default name of func.
        :param doc: description of the func.default docstring of func.

        """
        command = Command(func, doc)
        name = name or func.__name__
        self._commands[name] = command