Add exercise ratings
Add exercise ratings

For a first pass at this, I've copied ratings from other repos, (mainly
the Elixir repo, as it's the closest language to Clojure that I can
think of).

I'm sure this isn't perfect, but I think it's a decent first start, and
that it's better than rating everything as 1.

querenker/xclojure,querenker/xclojure,exercism/xclojure,exercism/xclojure