[FEATURE] Add EnclosureViewHelper

Used to obtain image information from a feed item.
Usage in template:
`<img src="{feed:item.enclosure(attribute:'url')}" />`

Related: #23

Author: buepro

Classes/ViewHelpers/Item/EnclosureViewHelper.php

@@ -0,0 +1,49 @@
+<?php
+namespace Fab\RssDisplay\ViewHelpers\Item;
+
+/*
+ * This file is part of the Fab/RssDisplay project under GPLv2 or later.
+ *
+ * For the full copyright and license information, please read the
+ * LICENSE.md file that was distributed with this source code.
+ */
+
+use TYPO3Fluid\Fluid\Core\ViewHelper\AbstractViewHelper;
+
+/**
+ * A View Helper which returns the enclosure of a SimplePie item.
+ */
+class EnclosureViewHelper extends AbstractViewHelper
+{
+
+    public function initializeArguments()
+    {
+        $this->registerArgument('attribute', 'string', 'The attribute to be obtained', true, 'url');
+        $this->registerArgument('key', 'int', 'The enclosure item key. Starts with 0.', false, 0);
+    }
+
+    public function render() {
+        $attribute = $this->arguments['attribute'];
+        if (!in_array($attribute, ['url', 'length', 'type'], true)) {
+            $message = sprintf('Attribute "%s" is no valid enclosure attribute.', $attribute);
+            throw new \InvalidArgumentException($message, 1500032671);
+        }
+        /** @var SimplePie_Item $item */
+        $item = $this->templateVariableContainer->get('item');
+        /** @var SimplePie_Enclosure $enclosure */
+        $enclosure = $item->get_enclosure($this->arguments['key']);
+        if ($enclosure) {
+            switch ($this->arguments['attribute']) {
+                case 'url':
+                    return $enclosure->get_link();
+                case 'length':
+                    return $enclosure->get_length();
+                case 'type':
+                    return $enclosure->get_type();
+                default:
+                    return '';
+            }
+        }
+        return '';
+    }
+}