add 'updated' field to post headers

_layouts/post.html

@@ -16,6 +16,12 @@ layout: default
         {{ page.date | date: "%b %-d, %Y" }}
       </time>
       •
+      {%- if page.updated %}
+      <time datetime="{{ page.updated | date_to_xmlschema }}" itemprop="dateModified">
+        (Updated {{ page.updated | date: "%b %-d, %Y" }})
+      </time>
+      •
+      {% endif -%}
       <span itemprop="about"><em>{{ page.description }}</em></span>
     </div>
   </header>

_posts/2017-9-6-brian-bars.md

@@ -3,6 +3,7 @@ title: Brian Bars
 description: >-
     Cheap and easy to make, healthy, vegan, high-carb, high-protein. "The Good
     Stuff".
+updated: 2018-01-18
 ---
 
 It actually blows my mind it's been 4 years since I used this blog. It was