Adding hjson extension.

I have been using this relatively new superset of JSON which I find to be a really nice middle ground between JSON and YAML. It reduces the effort of building objects and anyone who understands JSON can transition to. YAML tends to throw a wrench in the works when it needs to be edited outside of something like vim.

https://github.com/laktak/hjson

I'd love to get it in here so that I can give `mime-type` a go.