Add support for pre-static middleware

Some middleware needs to be executed prior to express.static.  This pull request adds support for that, and mitigates the need for [app.stack abuse](http://www.exratione.com/2013/03/nodejs-abusing-express-3-to-enable-late-addition-of-middleware/).

Use case: Less-middleware will never recompile .less files in static directories unless it's listed above express.static in the middleware stack.

For reference: https://github.com/emberfeather/less.js-middleware#troubleshooting