New option: closeEffect

I created a new option called closeEffect. As the name says, it allows you to define an effect to use when closing the lightbox.

I also added some special behavior when setting the appearEffect to "show" or the closeEffect to "hide". Rather than obeying the lightboxSpeed and overlaySpeed settings, it instantly shows or hides the lightbox. This is to support the case where you want to show it immediately (necessary for setting the focus on a form element within the lightbox in iOS or Android) but still want to have a closing effect.