Add support for post last updated

Author: rcvalle

_includes/post.html

@@ -1,6 +1,6 @@
 <article itemscope itemtype="http://schema.org/BlogPosting">
   <h1 itemprop="headline"><a href="{{ include.post.url | relative_url }}">{{ include.post.title }}</a></h1>
-  <p>{% if include.post.author %}By <span itemprop="author">{{ include.post.author }}</span> on {% endif %}<time itemprop="datePublished" datetime="{{ include.post.date | date_to_xmlschema }}">{{ include.post.date | date: "%B %-d, %Y" }}</time>{% if include.post.comments %}. <a href="{{ include.post.url | relative_url }}#disqus_thread" data-disqus-identifier="{{ include.post.url | relative_url }}"></a>{% endif %}</p>
+  <p>{% if include.post.author %}By <span itemprop="author">{{ include.post.author }}</span> on {% endif %}<time itemprop="datePublished" datetime="{{ include.post.date | date_to_xmlschema }}">{{ include.post.date | date: "%B %-d, %Y" }}</time>{% if include.post.updated %}. Last updated on <time itemprop="dateModified" datetime="{{ include.post.updated | date_to_xmlschema }}">{{ include.post.updated | date: "%B %-d, %Y" }}</time>{% endif %}{% if include.post.comments %}. <a href="{{ include.post.url | relative_url }}#disqus_thread" data-disqus-identifier="{{ include.post.url | relative_url }}"></a>{% endif %}</p>
   <div itemprop="articleBody">
     {% if page.type == "home" and site.excerpts %}
       {{ include.post.excerpt | markdownify }}