Added an optional environment variable S3_SERVER support

[AlexDumanskiy]

S3 is not limited to Amazon, there are third-party products, like enterprise storage systems, that provide it. There are also smaller open source projects like Minio that implement S3.

I needed your script to work with a local Minio server, decided to make a PR with my humble changes.

[scaleoutsean]

I wouldn't force path-style URLs on all S3 implementations without a switch/option, as most implementations can do both.