add \Slim\Router\::getRouteParam

Since now the Route Param can only(?) be accessed by the param of the Route Callback function.
It may be a little bit difficult to get the Route Param in the Route Middleware now, I think there should be another way to do the trick like other thing.
This patch offers another way to get Route Param, with this, we can get route param like the following code
```php
$app->get('/:param1/:param2', function (/*optional, add as you want*/) use ($app) {
    $param1 = $app->router->getRouteParam("param1");
    $param2 = $app->router->getRouteParam("param2");
});
```