Caddy Support

[yroc92]

So glad to see a SQL implementation for Certmagic! I want to use this as a Caddy module. It looks like all that is needed is to

1. satisfy an interface: https://caddyserver.com/docs/extending-caddy
2. add some module meta-data

A great example is the DynamoDB certmagic storage module: https://github.com/silinternational/certmagic-storage-dynamodb

I'm a novice at Go, but I'm interested in taking a shot at this.