performance improvement

• [!] Leverage express-state's cache option to improve expose() performance.

If changes to the YUI configuration need to be made after the first request (and we highly recommend NOT to do that), the app's YUI config will have to be re-exposed, e.g.:

res.expose(app.yui.config(), 'window.YUI_config');

ideally, you will just expose a piece of it per request instead of everything, this will allow you to overrule something defined at the app level, e.g.:

res.expose(true, 'window.YUI_config.fetchCSS');