Single Middleware type implementation

(Following up on http://help.slimframework.com/discussions/suggestions/581-enhancing-route-mdidleware)
(Please forgive me if something is not "as it should" but this is my very first Git usage and PR)

Changed Route.php to support the application middleware
(\Slim\Middleware instances).
This way:
- 1 single middleware concept and implementation
- "route middleware" can be execute code also after the route callback

$route = $s->get('/bar', $mw1, $mw2, $callable)
still works but requires that $mw1 and $mw2 are instances of
\Slim\Middleware.