load(template) {
                // Render and return template as a simple string, see https://github.com/twigjs/twig.js/pull/348 for more information
                if (!viewOptions || !viewOptions.allowAsync) {
                    fn(null, String(template.render(options)));
                    return;
                }

                template.renderAsync(options)
                    .then(out => fn(null, out), fn);
            }
Provide an extension for use with express 3.

@param {string} path The location of the template file on disk.
@param {Object|Function} The options or callback.
@param {Function} fn callback.

@throws Twig.Error