Adds option to limit the selection, and 2 related callback functions

[Frique]

Option "selectionLimit" with 0 for unlimited or (int) for a limit. options.limitReached() is called when reaching the limit, options.limitBlocked() is called when trying to add more than the limit. Also works for adding entire group and select_all, but select_all has a new param to respect or ignore the limit.

[refringe]

Been using this for a while now. Works great!

[petanne]

Perfect! Thanks a lot.