posts: Improve microdata for date published and date modified (#570)

layouts/partials/article_metadata.html

@@ -6,7 +6,8 @@
     {{ if ne $.Params.Lastmod $.Params.Date }}
         {{ i18n "last_updated" }}
     {{ end }}
-    <time datetime="{{ $.Date }}" itemprop="datePublished dateModified">
+    <meta content="{{ $.Params.Date }}" itemprop="datePublished">
+    <time datetime="{{ $.Params.LastMod }}" itemprop="dateModified">
       {{ $.Lastmod.Format $.Site.Params.date_format }}
     </time>
   </span>