Implement option to include the <optgroup> label in the selected option(s)

This just implements an option to show the label from the `<optgroup>`, in the case of a grouped select. Hopefully it's self-explanatory, otherwise. [[Demo]](http://rtlong.github.com/chosen/)

![Screenshot](http://dl.dropbox.com/u/18713714/chosen_include_group_labes.png)
![Screenshot](http://dl.dropbox.com/u/18713714/chosen_include_group_labels_multiple.png)