Allow custom S3 backend

[bbailleux]

In a professional environment, it is common to have access to an "on premise" back-end with S3 API (e.g. see https://geekflare.com/self-hosted-s3/).

The s3-bash4 tool only allows to access AWS instance, thus is not usable in such private infrastructure.

Could you add a way to specify the S3 domain instead of amazonaws.com so that the script can be used in a private context?

TIA

[bbailleux]

To @cvle : This issue could be closed if PR #8 is merged

[fabiog1901]

+1 @cvle