Add option for restoring the original positions upon disable.

This is quite useful when your page has multiple sections where you won't be seeing all of them at the same time.

In this case you wouldn't want the entire set of plax layers to be enabled all the time, and as such the clearLayers option is great.

However, moving back and forth from different sections of the page, there's a risk the objects aren't in their original position when clearing the layers, and when moving back and re-enabling plax on them, they would get a new initial position, which would be stacking up for each time you move back and forth to the section.

New option simply makes sure that each layer's original position is restored upon disable,  normally combined with clearLayers, but there are possible uses without clearing the layers as well.