Fixes and development

- `$parameter` argument in `findUrl` method now supports both string,
array and null value to avoid confusion.

- Using return value in callbacks now displays value (issue: #257)

src/Pecee/SimpleRouter/SimpleRouter.php

@@ -49,7 +49,7 @@ class SimpleRouter
      */
     public static function start()
     {
-        static::router()->routeRequest();
+        echo static::router()->routeRequest();
     }
 
     /**