Add markdown support to post body

Implemented markdown support in the post body to enhance the
editor's functionality. The main blog view now renders markdown
content as HTML. This change allows users to format their posts
using markdown syntax, providing a richer text editing experience.

- Imported the markdown module in blog.py.
- Converted post bodies from markdown to HTML in the index view.
- Updated templates to indicate markdown support in the post body
  field.
- Ensured safe rendering of HTML content in the blog index view.

Fixes #FLAS-29

flaskr/templates/blog/index.html

@@ -19,7 +19,7 @@
           <a class="action" href="{{ url_for('blog.update', id=post['id']) }}">Edit</a>
         {% endif %}
       </header>
-      <p class="body">{{ post['body'] }}</p>
+      <p class="body">{{ post['body']|safe }}</p>
     </article>
     {% if not loop.last %}
       <hr>