New sort option: mtime

Sort filelist by modification time, newer files first. Useful for
perusing a directory of images by most recently added or changed.

I had a shell wrapper that pre-sorted the filelist by mtime before
feeding to feh for "tailing" my image directories, so I thought it would
be useful to add this functionality directly to feh.

I did not add any tests, keybindings, or menu actions, but I can if you
like.

Thank you for the awesome software.