md5sum, needed additional feature

[Frank Klemm]

When using md5sum to compare files, the following feature can be useful to
increase speed.

md5sum --size files

prepends another 12 or 16 characters before the md5sum. This characters are
the hexadecimal representation of the file length. When comparing files,
first the file length can be compared and there is no need to compare the
md5sum if the length differs.

This is especially useful if
  - files mostly differs in size   and
  - files are a little bit larger (Audio, HQ Images, Video)
so that you don't need to read most of the huge files.

-- 
Frank Klemm