Add #slurp to slurp local files as strings

[orestis]

Like #include, but instead of expecting a config, just pass the raw string contents of a file.

E.g:

(defmethod reader 'slurp
  [{:keys [resolver source] :as opts} tag value]
  (slurp (if (map? resolver)
           (get resolver value)
           (resolver source value))))