add human sorting of files and a few more command line options

When images are named with the convention: file_1.png, file_2.png, file_10.png, file_14.png, etc. python will sort this as file_10.png, file_14.png, file_1.png, file_2.png. You must force it to do "natural" or "human" sorting. Of course a simple work-around is to zero-pad the filename as file_0001.png, file_0010.png, file_0014.png, etc. (but I never remember to do this).