Added twig filter (#27)

Author: Nyholm

src/Twig/DummyExtension.php

@@ -86,6 +86,7 @@ public function getFilters()
             new TwigFilter('inline_css'),
             new TwigFilter('markdown_to_html'),
             new TwigFilter('inky_to_html'),
+            new TwigFilter('serialize'),
         ];
     }
 }