Add possibility to run script before doing actual backup

[chrl]

Is your feature request related to a problem? Please describe.
I'm doing backups from MySQL using xtrabackup. I love how Walrus is able to do a tar archive of file, but first I need to create a dump with binlogs by running innobackupex /target/dir/.

Describe the solution you'd like
I'd like to have a field called "Before Script" to be able to run any arbitrary script before doing the actual backup.

Additional context
This can also be helpful for sending notifications that backup has started, or even emails. Or, during local development on my mac, I'd use /usr/bin/osascript -e 'display notification "Backup is starting" with title "Walrus" sound name "Submarine"' to show notification in Notification Center. Actually lots of usecases.