Add CloudFront support

At the moment the S3 uploader assumes URLs will be of the format:

public_endpoint + "/" + bucket + "/" key

This can be seen in the get_file method, and it seems hardcoded.

If there was another config option to turn off appending the bucket to URLs, it'd be trivial to use CloudFront as a CDN in front of the S3 bucket. Or alternately, using S3's built-in static website hosting would be much simpler.

Happy to write a PR if it helps, but be forewarned I've never written a line of Elixir in my life so it might be Actually Very Bad™ if I do that. 😅