docs: def init_app(self, app):
      """ Add ourselves into the app config and setup, and add a jinja function test """
  
      app.config.setdefault(FEATURE_FLAGS_CONFIG, {})
      app.config.setdefault(RAISE_ERROR_ON_MISSING_FEATURES, False)
  
      if hasattr(app, "add_template_test"):
        # flask 0.10 and higher has a proper hook
        app.add_template_test(self.check, name=self.JINJA_TEST_NAME)
      else:
        app.jinja_env.tests[self.JINJA_TEST_NAME] = self.check
  
      if not hasattr(app, 'extensions'):
        app.extensions = {}
      app.extensions[EXTENSION_NAME] = self