add hash apps

[wookietreiber]

in contrast to the *sum apps, these just print the hash

usage in pipes:

$ echo 'some string' | md5
8946014822a005f6027fb09bee776a86

also works on files directly:

$ md5 file
8946014822a005f6027fb09bee776a86

by just outputting the hash, it can be assigned to variables more easily:

hash=$(... | md5)