add getParam for router, now we can get Route Param anywhere

I can't using Route Param Since now I can only(?) access Route Param via the Route Callback Parameters.
This patch fixed this problem.
e.g.

```php
$app->get('/:param1/:param2', function (/*now it's optional*/) use ($app) {
    $param1 = $app->router->getParam('param1');
    $param2 = $app->router->getParam('param2');
    $allparam = $app->router->getParam();
});
```