Enables setting a center when crop resizing

Currently you can anchor a resize-crop with top,right,left,etc, this does somewhat the same thing, but with a coordinate set.

Passing in ?center=0.3,0.1 the crop will try to adjust the crop to center on this point. setting the center of the crop 30% from the top and 10% off the left side, it detects if it goes beyond the edges and anchors the crop to the side. 

It will still respect the way resizing is done - with only cropping on either x or y so the offsetting will only affect one of these axis.