Restricting only the smaller dimension when resizing #30

This is a great plugin! I have a suggestion that might make it even more useful.

I often need to resize an image while restricting only the smaller of the two dimensions, similar to the behavior of background-size: cover in CSS. Since you don't necessarily know whether the image is portrait or landscape, you can't simply use the maxWidth or maxHeight options to accomplish this. This would require a new option that would look something like this:

smallDimension: 500

In this case, if the image is portrait, the width would be constrained to 500 pixels. If the image is landscape, the height would be constrained to 500 pixels.