Add option for outputting image dimensions

[kpoelhekke]

First off: great script! Love it! It would be a really nice feature though if Spritemapper could optionally output the original image dimensions. That way I would have to manually check and set them myself.

So for example:

example{

background: url('images/sprite.png') no-repeat 0 -186px;
width: 45px;
height: 30px;
}