Add multiple region specification via configuration variables

[sopel]

This has been triggered by #44 and depends on #45.

Supporting the specification of multiple regions will be useful for every day operation and might also help to evaluate whether cross region support could be integrated into the AWS CLI directly instead.

• ⚠️ Obviously care must be taken to ensure this won't break the AWS CLI in turn, a respective patch should be submitted in case.

[sopel]

ℹ️ worth noting that boto has just received a most welcome refactoring to facilitate JSON-powered endpoints (see boto/boto#2045), with the explicit notion to make overriding/extending the regions/endpoints Boto has available much easier - at first sight this seems to be restricted to merge_endpoints() only right now and doesn't support region exclusion desired/required for e.g. #53 yet, but that might be easier to add now at least.