loader patch hooks

Provide hooks to add patches for YUI on both the client and server.

This will facilitate to patch YUI, and its loader easily. Here is an example:

```
app.yui.patch(require('express-yui/lib/patches/optional-requires'));
```

by calling that while setting up the express application, you will get a new feature in YUI to allow specifying `optionalRequires` in the 4rd argument of `YUI.add()` to specify modules that should be required only if they are available in the runtime. That's because the patch implemented in `express-yui/lib/patches/optional-requires.js` modifies the way the loader function to support this.