Add option for greedy crop

[trlkly]

When it comes to removing borders on images, it's often actually better to lose a little bit of data on the left and top than to leave the border in. An option that would say it's okay to lose a few pixels in height and width would be useful for this situation.

As it is right now, I have to intentionally overcrop and then check to see if I correctly guessed the block size. All the other programs I've found that do a lossless JPEG crop allow for this "greedy" option.

[danmichaelo]

Depends on #39