Add url as option to text type.

REQUEST_FORMAT.md

@@ -16,7 +16,8 @@ and type-dependent data. e.g.
 ```
 {
     "type": "text",
-    "text": "This will hopefully be useful information."
+    "text": "This will hopefully be useful information.",
+    "url": "link_to_external_page_with_more_information"
 }
 ```