adding extend and augment methods off from express-yui module

This allows others to extend express or augment an app instance manually.

Sometimes you have a 3rd party module that is requiring
`express`, and even creating the app under the hood, in which case you can just augment
an existing express app instance by doing this:

```
var yui = require('express-yui'),
    express = require('express'),
    app = express();

// calling a yui static method to augment the `express` app instance
yui.augment(app);

app.yui.applyConfig({ foo: 'something' });
```

This also helps when using `express-yui` thru `npm link`.