Use nrepl options middleware

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]})
```

Is this how Leiningen does it? It looks a bit strange.