External ID support for assume-role switch

[iancward]

PR #667 added the ability to assume a role via the command line; however, it is best practice to use an external ID when assuming roles between accounts, so it would be nice to be able to do this on the command line.

Also, the assume role CLI option isn't documented in the README; it would be nice if there was some info showing that it was a feature.

[ekristen]

@iancward this was implemented on my fork of aws-nuke, which has recently become the active successor to this project. It was implemented via ekristen/aws-nuke#52

If you should run into any issues, please open an issue over there.

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