Add a plus sign to the output_groups option

For people upgrading from earlier versions of rules_oci, it works out better to use `+tarball` than `tarball`. The latter will break builds using the older version of rules_oci, making it harder to upgrade incrementally.

It's probably a good habit, anyway. When you want an extra output group to a build, you usually don't want to cancel all the output groups you don't know about. You just want to add the one new one that you do care about.