`delete-bucket` command

[simonw]

I already have delete-user - this would be a similar utility but for deleting buckets. Mainly so I don't have to remember how to do it with awscli.

[simonw]

A already wrote some code for doing this in a test here: https://github.com/simonw/s3-credentials/blob/56cf3794e6408a8a10476f07876d81be558bcdc4/tests/test_integration.py#L162-L174

[mileswwatkins]

This addition to the CLI would be much appreciated, whether as a new subcommand or alternatively as part of the delete-user subcommand (as a --delete-bucket flag, paralleling the s3-credentials create BUCKET_NAME --create-bucket pattern). Thanks for developing s3-credentials!