snogdiff: Compute and print image-comparison statistics
snogdiff: Make output image optional
snogdiff: Add image-equality-threshold options

New options -d/--delta and -m/--mse options allow the threshold where
images are considered identical to be specified (by default, any
difference at all is considered "non-identical").

Comparison statistics are only printed for non-identical images, and
the exit value of the program is "success" only for "identical" images.
snogdiff: Add -q/--quiet option