Add function addCell

Author: Franck

src/PhpWord/Element/AbstractContainer.php

@@ -87,7 +87,7 @@ public function __call($function, $args)
             'ListItem', 'ListItemRun', 'Table', 'Image', 'Object', 'OLEObject',
             'Footnote', 'Endnote', 'CheckBox', 'TextBox', 'Field',
             'Line', 'Shape', 'Title', 'TOC', 'PageBreak',
-            'Chart', 'FormField', 'SDT', 'Comment',
+            'Chart', 'FormField', 'SDT', 'Comment', 'Cell'
         ];
         $functions = [];
         foreach ($elements as $element) {