Add only new installed/supported app configs to backup about mackup

[joshmedeski]

That's a good idea. I would also like to see something like this. The project hasn't had any new features for some time, so we'd have to probably implement something like this ourselves.

I think a --only-new or --skip-existing flag would make it easier to scan for just applications that haven't been backed up before. I'd also love to see a summary of what got backed up after the script completes.

Hopefully mackup --dry-run --verbose backup can help in the mean time. It will list items like "is already backed up" to give you some insight without actually running the backup.