Added thumbnail support for blog posts (alshedivat#1371)

Implemented alshedivat#425 based on given code.


![image](https://user-images.githubusercontent.com/31376482/234644266-07c1aebd-6a83-482f-8ac2-846fe5c0653d.png)

---------

Signed-off-by: George Araujo <george.gcac@gmail.com>

_posts/2015-05-15-images.md

@@ -5,6 +5,7 @@ date: 2015-05-15 21:01:00
 description: this is what included images could look like
 tags: formatting images
 categories: sample-posts
+thumbnail: assets/img/9.jpg
 ---
 This is an example post with image galleries.
 

blog/index.html

@@ -53,7 +53,11 @@ <h2>{{ site.blog_description }}</h2>
     {% assign categories = post.categories | join: "" %}
 
     <li>
-      <h3>
+{%- if post.thumbnail -%}
+<div class="row">
+          <div class="col-sm-9">
+{%- endif -%}
+        <h3>
         {% if post.redirect == blank %}
           <a class="post-title" href="{{ post.url | prepend: site.baseurl }}">{{ post.title }}</a>
         {% else %}
@@ -95,6 +99,13 @@ <h3>
               {% endfor %}
           {% endif %}
     </p>
+{%- if post.thumbnail -%}
+    </div>
+  <div class="col-sm-3">
+    <img class="card-img" src="{{post.thumbnail | relative_url}}" style="object-fit: cover; height: 90%" alt="image">
+  </div>
+</div>
+{%- endif -%}
     </li>
 
     {% endfor %}