Add possibility for reuse to defresource

Adding the possibility for reuse as discussed on the mailing list

see https://groups.google.com/forum/#!topic/clojure-liberator/CKl2lPp-3ZY

example usage:

```clojure
;; still compatible with the old format
(defresource without-param
  :handle-ok (fn [_] (format "The text is %s" "test")))

(defresource parameter [txt]
  :handle-ok (fn [_] (format "The text is %s" txt))
  :available-media-types ["application/xml"])

;; how to introduce reuse
(def standard-config
  {:available-media-types ["application/json"]})

(defresource with-options
  standard-config
  :handle-ok (fn [_] (format "The text is %s" "this")))

(defresource with-options-and-params [txt]
  standard-config
  :handle-ok (fn [_] (format "The text is %s" txt))
  :available-media-types ["application/xml"])  ;; this actually overrides the standard-config

;; you can also add a parametrized config
(defn parametrized-config
  [media-type]
  {:available-media-types [media-type]})

(defresource with-options-parametrized-config [media-type txt]
  (parametrized-config media-type)
  :handle-ok (fn [_] (format "The text is %s" txt)))
```