Add structured data markup for itemprop="comment"

- Close #458

_includes/comment.html

@@ -1,22 +1,22 @@
-<article id="comment{{ include.index }}" class="js-comment comment">
+<article id="comment{{ include.index }}" class="js-comment comment" itemprop="comment" itemscope itemtype="http://schema.org/Comment">
   <div class="comment__avatar-wrapper">
     <img class="comment__avatar" src="https://www.gravatar.com/avatar/{{ include.email }}?d=mm&s=80">
   </div>
   <div class="comment__content-wrapper">
-    <h3 class="comment__author">
+    <h3 class="comment__author" itemprop="author" itemscope itemtype="http://schema.org/Person">
       {% unless include.url == blank %}
-        <a rel="nofollow" href="{{ include.url }}">{{ include.name }}</a>
+        <span itemprop="name"><a rel="external nofollow" itemprop="url" href="{{ include.url }}">{{ include.name }}</a></span>
       {% else %}
-        {{ include.name }}
+        <span itemprop="name">{{ include.name }}</span>
       {% endunless %}
     </h3>
     <p class="comment__date">
       {% if include.date %}
-        {% if include.index %}<a href="#comment{{ include.index }}">{% endif %}
-        <time datetime="{{ include.date | date_to_xmlschema }}">{{ include.date | date: "%B %d, %Y at %I:%M %p" }}</time>
+        {% if include.index %}<a href="#comment{{ include.index }}" itemprop="url">{% endif %}
+        <time datetime="{{ include.date | date_to_xmlschema }}" itemprop="datePublished">{{ include.date | date: "%B %d, %Y at %I:%M %p" }}</time>
         {% if include.index %}</a>{% endif %}
       {% endif %}
     </p>
-    {{ include.message | markdownify }}
+    <div itemprop="text">{{ include.message | markdownify }}</div>
   </div>
 </article>