Support Custom S3 Endpoint, etc.

[adamgoose]

We are hosted on GCP, and Google Storage has an S3-Compliant API. We also use open-source alternatives like Minio.

I'd be useful to specify more (if not all) S3 configuration options via your config file. This would allow us to use dbacked with Minio.

[aslafy-z]

I'd recommend using minio-go library: https://github.com/minio/minio-go