Add the nextImageToPreloadOption

Hi!

In response to the feature request #834 I tried to purpose a nextSlidesToPreload option.
I had a specific issue on my project that I illustrated here in this JsFiddle with the new option: http://jsfiddle.net/dmhzc360/1/

I don't know if this is the best way to do that but it respond to my need and anyway, i would be glad to discuss other options!
Thanks a lot for this plugin!
Juliette