add unique option

Author: mayeco

Annotation/StringGenerator.php

@@ -28,6 +28,11 @@ class StringGenerator extends Annotation
      */
     public $length = 8;
 
+    /**
+     * @var boolean
+     */
+    public $unique = true;
+
     /**
      * @var string
      */
@@ -37,4 +42,4 @@ class StringGenerator extends Annotation
      * @var string
      */
     public $prefix_callback;
-}
+}