Create explicit argument for bucket region

[stevefan]

Please specify whether your issue is about:

• [x] a suggested code or documentation change, improvement to the code, or feature request

Without taking a deep dive in the code, it seems like aws.s3 Checks bucket region using get_location() regardless if user determined values are provided. Specifying the region option seems to be used only by aws.signature.

Because I had not attached a GET Bucket location permission, the command failed mysteriously until I used the verbose option.

I suggest creating an option, bucket_region, that takes a default argument of aws::get_location(bucket). Previous code would be unaffected and it would give a clue why it might not be just working.