Add GraphQL API

[hendursaga]

I'm legitimately surprised certstream doesn't use GraphQL yet! I'm not sure what others are using certstream for, but for a project or two I had in mind, most of the data returned from the JSON API will just be discarded. One advantage of GraphQL, among others, is that you can ask it for the data you need, but no more, instead of wasting CPU, memory, and, of course, network bandwidth. Then there's typings, and built-in API documentation, and more!

Absinthe seems to be the best GraphQL library for Elixir these days.