Add visible option to allow multiple items visible simultaneously

This feature allow more than one item be visible simultaneously. 
There are a lot of use cases for it, such as a list of products, gallery images thumbnails, etc...

The implementation checks the items length and visible option value, and prevents the slider of sliding if the last item is yet visible. If loop option is true, it slides to the first item.

Conflicts:
	src/unslider.js