Initialize the plugin.

def setup(self, app):
        """Initialize the plugin."""
        super().setup(app)

        if self.cfg.secret == 'InsecureSecret':
            app.logger.warning(
                'Use insecure secret key. Change SESSION_SECRET option in configuration.')

        self._user_loader = asyncio.coroutine(lambda id_: id_)  # noqa

        app.on_response_prepare.append(self.save)

        if self.cfg.auto_load:
            app.middlewares.append(self._middleware)