fix(layouts/single): Use <time>

layouts/_default/single.html

 {{ define "main" -}}
 <div class="post">
   <h1>{{ .Title }}</h1>
-  <span class="post-date">{{ .Date.Format "Mon, Jan 2, 2006" }}</span>
+  <time datetime={{ .Date.Format "2006-01-02T15:04:05Z0700" }} class="post-date">{{ .Date.Format "Mon, Jan 2, 2006" }}</time>
   {{ .Content }}
 </div>
 
 <h2>Comments</h2>
 {{ template "_internal/disqus.html" . }}
 {{- end }}
-{{- end }}
+{{- end }}