[RFE] Add option to call external command for backup status

[maxxer]

We use Zabbix, it would be great to be able to call zabbix_sender once the backup is done with the exit status, so we can log directly into the monitoring system if it was successful or not. Monitoring email is inconvenient (for us).

[kmwoley]

I like this idea. I'll work on something like this for the next revision.