Added support for configuring AWS S3 CNAME.

[paulofreitas]

So I have came up with another feature request! :)

This enables us to use a custom previously-configured CNAME (Amazon CloudFront CDN) to be used when building URLs, thus serving the files with a custom domain.

Hope you like it! :+1:

[dmyers]

Sorry for the delay, trying to test it out a bit. I like the idea as I could see myself using it in the future too.

The only thing I am curious about is if there is a way to make the getObjectUrl() call return the correct URL somehow by passing config options to S3Client. I looked around online and found a reference in the old PHP SDK and I believe it changed the 'base_url' config and set the 'bucket' to null and I sort of have it working, but the URL is wrong.

I made an issue on the AWS PHP SDK to ask if it is still possible.