Initialize cache instance.

def init_extension(self, app):
        """Initialize cache instance."""
        app.config.setdefault('CACHE_VERSION', '0')
        app.config.setdefault('CACHE_PREFIX', 'r')
        app.config.setdefault('CACHE_BACKEND', 'rio.exts.flask_cache.NullBackend')
        app.config.setdefault('CACHE_BACKEND_OPTIONS', {})