Added feature to preload images of only initially visible slides

Currently the script preloads all slides images before starting the slider. This isn't very optimal for user experience as he keeps seeing the loader for a considerable time.

The most common usage of the slider would be to show slideshow with fixed sized images with one slide at a time. In this scenario all required calculations can be done right after the image for first slide is loaded and the slider can be started.

So I added a new feature and related setting option `preloadImages`. With default value "all" you get the current behavior of preloading all images. But you could set it to "visible" which will cause the slider to start after the images for all initially visible images to be loaded.