Add reference to the createComment parameters

I've only seen the reference column being used by integrations (gitbucket,etc), would be nice that from the API we could also use that column for other references.

app/Api/Comment.php

@@ -25,12 +25,13 @@ class Comment extends \Kanboard\Core\Base
         return $this->comment->remove($comment_id);
     }
 
-    public function createComment($task_id, $user_id, $content)
+    public function createComment($task_id, $user_id, $content, $reference = '')
     {
         $values = array(
             'task_id' => $task_id,
             'user_id' => $user_id,
             'comment' => $content,
+            'reference' => $reference,
         );
 
         list($valid, ) = $this->comment->validateCreation($values);