Support external-ids when authenticating by STS:AssumeRole

[leftathome]

Some users may want to use STS to get temporary credentials to run aws-nuke, and in some cases this may mean providing an External-ID header for the STS:AssumeRole action to get credentials.

(See AWS documentation here )

This pull request adds support for the --external-id flag in the CLI, passes the flag for client authentication only when provided and includes documentation for its use in README.

I've been using this code on an internal fork for a few months with no problems.