support an rsync --delete-excluded flag

It appears the gsutil rsync -x flag (exclude), combined with -d (delete) will exclude operations on files that are in the source location and match the exclusion parameters. However, there is no way to delete the files at the destination, if they are copied, but then later added to the exclusion criteria.

Rsync has a flag --delete-excluded that adds this functionality and it would be great to see in gsutil rsync.

Thanks,