Modular caching and CDN support

[danielhartnell]

I wanted to create a GitHub issue as a follow up to our office hours discussion about caching. Our team would be interested in CloudFront in some cases and it would be nice if we could enable this in Terraform.

We would like to be able to enable a CDN like CloudFront if it's appropriate for our website (maybe we want to reduce request latency around the world). In our discussion it sounds like we might be able to set a flag in Terraform to say that we want caching and we want that through some arbitrary caching layer like Varnish or a CDN like CloudFront.