Add `-region=<region_name>` argument.

We're currently forced to use the `AWS_REGION` environment variable to set/override AWS region where pretty much everything else is available via CLI argument, so just adding this for the sake of consistency and user experience.

Start by locating the CLI argument parsing and the existing AWS_REGION handling in the Go code. Add the region argument consistently with the other CLI options, ensure it can set or override the AWS region, and verify that the command uses the supplied value.

Written by the indexing model from the issue text.