add chainable loader abstract

src/Loaders/ChainableLoaderAbstract.php

@@ -0,0 +1,23 @@
+<?php
+
+/*
+ * This file is part of YaEtl
+ *     (c) Fabrice de Stefanis / https://github.com/fab2s/YaEtl
+ * This source file is licensed under the MIT license which you will
+ * find in the LICENSE file or at https://opensource.org/licenses/MIT
+ */
+
+namespace fab2s\YaEtl\Loaders;
+
+/**
+ * Class ChainableLoaderAbstract
+ */
+abstract class ChainableLoaderAbstract extends LoaderAbstract
+{
+    /**
+     * This one is chainable
+     *
+     * @var bool
+     */
+    protected $isAReturningVal = true;
+}