Use nrepl options middleware

Utilize :nrepl-middleware for Ring's nREPL

Makes 'lein ring server' honor :nrepl-middleware specified in :repl-options, optionally overriden directly in the :ring option map.

Possibly conflicts with #118 or might supercede it - I don't use Cider.

Example usage (with LightTable nREPL middleware):

```clojure
;;project.clj

(defproject my-app "0.1.0-SNAPSHOT"
:ring {:handler my-app.core/handler
         :nrepl {:start? true
                 :nrepl-middleware [lighttable.nrepl.handler/lighttable-ops]}} ;; overrides below

;;or

:repl-options {:nrepl-middleware [lighttable.nrepl.handler/lighttable-ops]})
```

This could be rewritten as:

```clojure
     (load-namespaces
      'ring.server.leiningen
      (if (nrepl? project) 'clojure.tools.nrepl.server)
      (if (nrepl? project) (nrepl-middleware project))
      (-> project :ring :handler)
      (-> project :ring :init)
      (-> project :ring :destroy)))))
 ```

You don't need the `apply` and `let`, since `load-namespaces` filters out `nil` values.