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.

Patch by sdaau on IRC. Thanks!

Previously feh would set a black background around wallpaper images,
with no option to change.

This commit extends the functionality offered by the --image-bg option,
which is available for regular image viewing, to centered and max'ed feh
backgrounds. Essentially, this means a white border can now be chosen
in addition to a black one.

It used to save the thumbnail for /path/to/./pmage.png, now it is always
/path/to/image.png