dave/tim add #config/edn

@ataggart 
You'll never get here since Object and nil satisfy both protocols. This should be sufficient for the body:
```clojure
(let [s (extract e)]
  (if (string? s)
    (->EdnVar e (edn/read-string s) (provided? e))
    (throw (IllegalArgumentException. (str "Argument to #config/edn must be a string: " (pr-str e))))))
    
```