Add ourselves into the app config and setup, and add a jinja function test

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