Support for pre & post scripts

[paescuj]

It should be possible to set (several) pre and post scripts, which are called by the online-backup script.

Pre script:

• The online-backup runs only after this script is completed.
• The exit code of pre script should be evaluated and the online-backup should act accordingly.
• For example a database dump script.

Post script:

• This script runs after the online-backup is completed.
• For example a notification script.