Allow to jQuery.bind() on viewsSlideshowCycle.pager_before_fn()

There is a world if extensibility that could be open by just letting bind as requested, since this is actually hard to do right now.

Specifically, this new feature will allow to integrate with plugins like backstretch, so the slideshow cycle can trigger other plugins. For example: #2109767: Doesn't seem to work when used with views slideshow.