Added support for configuring AWS S3 CNAME.

So I have came up with another feature request! :)

This enables us to use a custom previously-configured CNAME ([Amazon CloudFront CDN](http://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/MigrateS3ToCloudFront.html)) to be used when building URLs, thus serving the files with a custom domain.

Hope you like it! :+1: