option to specify a script that runs after backups

This script could be used to sync backups to a central server, or just
to create a file to let external scripts know that backups are
(sucessfully) done and can be picked up.

The postscript option (defaults to false) and can be either a string or
an array of strings. These strings will be directly put into the
mysqlbackup.sh and as such can either be shell commands, or externally
managed files.