Support --download-api-only in bash-completion

Simple pull request - adds --download-api-only to bash-completion.

I was tempted to add a help message for it in commandline, but I can't actually find any suitable documentation to aptly describe what it does.. --disable-cpio-bulk-download could do with one too.

If someone has some suitable text I'm happy to push a commit to add those in (they're suppressed right now)