Added wkhtmltopdf stuff and created preview generator service

module/Common/src/Service/PreviewGeneratorInterface.php

@@ -0,0 +1,13 @@
+<?php
+namespace Shlinkio\Shlink\Common\Service;
+
+interface PreviewGeneratorInterface
+{
+    /**
+     * Generates and stores preview for provided website and returns the path to the image file
+     *
+     * @param string $url
+     * @return string
+     */
+    public function generatePreview($url);
+}