Dry-run / Interactive mode

Consider adding a `-n` flag to dangerous commands such as `beadm create`, `beadm activate`, `beadm destroy`, `beadm mount`, `beadm unmount`, `beadm rename`.

Personally, I'm always scared that something may go terribly wrong, and want to see what will be done first.

Perhaps a `-i` mode which could simply just run the `beadm` command with `-n`, then rerun after confirmation without `-n`.
