Allow to group exports

Export/GroupedExportInterface.php

@@ -0,0 +1,16 @@
+<?php
+/*
+ * 
+ */
+namespace Chill\MainBundle\Export;
+
+/**
+ * Add a grouping option to exports. 
+ * 
+ * **usage**: the exports will be grouped under the key given by the `getGroup` 
+ * method. 
+ */
+interface GroupedExportInterface
+{
+    public function getGroup(): string;
+}