gdocs shortcode: Support all doc types (Slides, Docs, Sheets, etc.)

- Shortcode syntax changed to: {{< gdocs src="https://..." >}}
 - Use with Google Docs `embed` URL rather than `link` URL.

 Close #544

layouts/shortcodes/gdocs.html

@@ -1,3 +1,3 @@
 <div class="responsive-wrap">
-  <iframe src="{{ strings.TrimSuffix (index (last 1 (split (.Get 0) "/")) 0) (.Get 0) }}embed?start=false&loop=false&delayms=3000" frameborder="0" width="960" height="569" allowfullscreen="true" mozallowfullscreen="true" webkitallowfullscreen="true"></iframe>
+  <iframe src="{{ .Get "src" }}" frameborder="0" width="960" height="569" allowfullscreen="true" mozallowfullscreen="true" webkitallowfullscreen="true"></iframe>
 </div>