Add App::keys() method

src/App.php

@@ -150,6 +150,18 @@ class App {
     );
   }
 
+  /**
+   * Get list of keys of a specific option
+   *
+   * @param string $key The configuration variable key
+   *
+   * @return array An array of the keys of a specific option
+   **/
+  public static function keys($key) {
+    $value = self :: get($key);
+    return (is_array($value)?array_keys($value):array());
+  }
+
   /**
    * Set a specific option value
    *