Specify an S3 URL for config file

Hi, would you be interested in a PR that would allow specification of an S3 URL from which to load config.edn?

Off the top of my head the main issue would be adding an amazonica dependency to this lib, which would be a good thing to avoid. If the functionality sounds like something you'd be interested in I'll have a think to see if there's a neat way to make S3 support opt-in only.