layouts: Improve usage of datetime

Use the date format from the config, and print it in a time tag.

See: https://golang.org/pkg/time/

layouts/_default/summary.html

@@ -1,6 +1,6 @@
 <div class="blog-post">
   <h2 class="blog-post-title"><a href="{{ .Permalink }}" title="{{ .Title }}">{{ .Title }}</a></h2>
-  <p class="blog-post-meta">{{ .Date.Format "Mon, Jan 2, 2006" }} by <a href="#">Mark</a></p>
+  <p class="blog-post-meta"><time datetime="{{ .Date }}">{{ .Date.Format .Site.Params.date_format }}</time> by <a href="#">Mark</a></p>
   {{ .Summary }}
   <a href='{{ .Permalink }}'><nobr>Read more →</nobr></a>
 </div> <!-- /.blog-post -->