Refactor region selection to facilitate regular expressions

This addresses #53 and adjusts #50 - it removes China and GovCloud regions by default (ensuring existing code will work unchanged) by refactoring the region selection to regular expression based substring selection and defaulting to global regions only:
```
  -r REGION, --region REGION
                        A regular expression based region substring selector
                        [default '(ap|eu|sa|us)-[enws]']
```

* :information_source:  This implies a minor incompatibility by removing the dedicated GovCloud arguments just introduced via #51 and would usually require a major version bump accordingly, but assuming this isn't actually used anywhere outside of well known contexts, nobody should be affected (and if so, it would be an easy fix).