Add external links for tasks with plugin api

app/Core/ExternalLink/ExternalLinkInterface.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace Kanboard\Core\ExternalLink;
+
+/**
+ * External Link Interface
+ *
+ * @package  externalLink
+ * @author   Frederic Guillot
+ */
+interface ExternalLinkInterface
+{
+    /**
+     * Get link title
+     *
+     * @access public
+     * @return string
+     */
+    public function getTitle();
+
+    /**
+     * Get link URL
+     *
+     * @access public
+     * @return string
+     */
+    public function getUrl();
+
+    /**
+     * Set link URL
+     *
+     * @access public
+     * @param  string $url
+     */
+    public function setUrl($url);
+}