Add post excerpts to the main page.

_includes/category_list.html

@@ -5,6 +5,7 @@
       <h2>
         <a class="post-link" href="{{ post.url | prepend: site.baseurl }}">{{ post.title }}</a>
       </h2>
+      {{ post.excerpt }}
     </li>
   {% endfor %}
 </ul>