Added callback for custom button being pressed

I've added ability to implement custom blocks or selectors when custom buttons are pressed.
IMO: When people want to set custom action for button - explicit behaviour to hide picker automatically after button pressed.
So, it's better to move `hidePickerWithCancelAction` inside of Picker logic and just dismiss picker and not call cancel action in this case.
Moreover, it's preferrable to call actionBlock only after picker already dismissed to avoid bugs, such #63 (when you want to show another picker immediately after first one is dismissed.

I'm ready to discuss it in details.