Add regions to the auto-complete

Would like to suggest adding the actual regions to the --region auto-complete (e.g. show us-east... and so on).

It would also be great to support the --region (and other options) in other places except after the service (e.g. also at the end).

Start by locating the --region autocomplete entry point and the option-parsing path for arguments after a service. Trace how region values are discovered and how options are positioned; done means regional suggestions appear and --region and other options work in the additional positions described, with coverage for both behaviors.

Written by the indexing model from the issue text.