Source bundle as URL/S3 path

[justinkillen]

Feature request.

Currently, helm-s3 assumes that the source bundle/artifact is on the local disk. In many common CI/CD scenarios, the artifact is built sometime in the middle and stored in a holding location and it isn't until much later that the artifact gets published out to it's final destination. It is generally simple to download the artifact to the local disk and then run helm-s3, but it would be nice if helm-s3 could just do it for me.

syntax examples:
helm s3 push s3://my-temp-bucket/helm_bundle.tgz coolcharts --relative
helm s3 push https://artifact-repository/mycoolproject/1.0/helm.tgz coolcharts --relative

bonus points if the S3 variant does a S3-to-S3 copy instead of download/upload