Add markdown support to post body

Implemented markdown support in the post body to enhance
text formatting capabilities. This change allows users to
enter markdown in the create/update interface and have it
rendered as HTML in the main blog interface. The markdown
library was imported and utilized to convert markdown text
to HTML in the blog index view. Additionally, updated the
form labels to indicate markdown support and ensured safe
rendering of HTML content in the blog index template.

Issue ID: FLAS-67

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>