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.
support mariadb

let's start by removing the 16 char username limit