Added support for the s3_host_alias (needed to use Cloudfront)

With this, you can configure your app to use Cloudfront (or a CNAME) for your attachment delivery.
Just add these to your configuration : 
```
config.cms.attachments.s3_bucket = "bucket-name"
config.cms.attachments.s3_host_alias = "12345.cloudfront.net"
config.cms.attachments.url = ":s3_alias_url"
```