Command phpdoc or other flag to allow third-party commands to run before WordPress is present

While working on #433 (Backup and Restore), I ran into the problem where it looks as if there's no way that I can restore from a compressed archive, or do any sort of pre-parsing of the restore contents before WordPress exists.

Now, it does occur to me that the feature suggestion is very much something that should only be used for getting WordPress in commission (like the existing core download and core config that work around this problem), but if backup/restore (in particular, restore) does not end up in core wp-cli, unless I'm missing something, this is the only way to work around it.

Related: #433