Add flag to specify an s3 compatible endpoint

[jbialy]

It think that it'd be great to have an option to specify an s3 compatible endpoint so that the sync client can be used with servers like Minio. The SDK doesn't support simply adding an endpoint as an environment variable (similar to AWS_REGION, etc) but it could be implemented as a configurable option to pass into aws.Config.

Example:
https://github.com/minio/cookbook/blob/master/docs/aws-sdk-for-go-with-minio.md