Add post.js only on post layout

Author: Claudio Cortese

_layouts/post.html

@@ -79,5 +79,7 @@ <h1 class="section-heading" data-text="{{ page.title }}" data-delay="{{ page.typ
     {% include modals.html %}
     {% include js.html %}
 
+    <script src="{{ "/js/post.js" | prepend: site.baseurl }}"></script>
+
     </body>
 </html>