adding `debug` middleware to control it per request

Trello:
https://trello.com/c/uZnI5ehS/184-express-yui-allow-customization-of-filter-and-combine-for-the-seed-per-request

This feature covers a very edge case where someone want to force the filter and the combine flag for the seed urls. For example, if you have a middleware to detect debug mode, you might want to customize that for a particular request by creating a middleware that exposes custom values. Here is an example of the debug middleware:

```
var express = require('express'),
       yui = require('express-yui');

// exposing yui into the client side for all requests
app.use(yui.expose());

// using debug when a querystring `?debug=1` is passed
app.use(function(req, res, next) {
    if (req.query.debug) {
        yui.debug()(req, res, next);
    } else {
        next();
    }
});
```

TODO

 * [x] make `yui.expose()` a `connect` and `express` compatible middleware
 * [x] example
 * [x] test