Save hooks and backups

I'm thinking about adding a way for Penn to run a specified program after every successful database save. This can be used along with scripts that check the databases (main, chat and mail) into a version control system, email them somewhere, or a combination -- email a full db on startup, and then a xdelta diff against that.

Not that none of this can't be done now using cron, but it seems more elegant to have it triggered on saves. And, of course, there's other possibilities -- using a compression program like pbzip2 that doesn't lend itself to piping, and things I haven't even thought of yet.