Add formatContainer handler and selectable handler

This pull request adds two handlers:
formatContainer - it gives you an ability to modify container element after select2 initialization
selectable - sometimes, you need a bit more complicated logic to define whether a result element selectable or not than just blank id.

Also, I'v added one more event (select2.results-loaded) which triggers when result of search rendered. 

These changes were necessary to implement custom scrollbar for select2. https://github.com/Termina1/select2-tinyscrollbar
Here you can find some kind of bridge between select2 and tinyscrollbar