Specify a broadway extension to initialise

        .. code-block:: python

            factory = Factory()

            factory.add_extension('broadway_sqlalchemy')

        :param extension: import path to extension
        :type extension: str

def add_extension(self, extension):
        """
        Specify a broadway extension to initialise

        .. code-block:: python

            factory = Factory()

            factory.add_extension('broadway_sqlalchemy')

        :param extension: import path to extension
        :type extension: str
        """
        instance = werkzeug.utils.import_string(extension)

        if hasattr(instance, 'register'):
            instance.register(self)

        self._extensions.append(instance)