S3: support for path style buckets

[flixr]

Right now the S3 backend doesn't support path style buckets.
This is needed to e.g. connect to minio when buckets use path style addressing instead of subdomains.

There is the s3ForcePathStyle option in https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/S3.html#constructor-property

I'm not really familiar with js/typescript nor this sdk, but I tried to add this option:
flixr@8047c62

Unfortunately this doesn't seem to be sufficient, it still tries to access it as bucket.endpoint instead of endpoint/bucket.