Add showCanonicalLink to markdown metadata (#428)

If an article has a `canonicalUrl` set, Originally published at `<hostname>` will be displayed at the top of the article by setting `showCanonicalLink` to true. 

Usage =>
  In front-matter:
    canonicalUrl: "<URL>"
    showCanonicalLink: true

layouts/_default/single.html

@@ -17,6 +17,7 @@
       {{- partial "post_meta.html" . -}}
       {{- partial "translation_list.html" . -}}
       {{- partial "edit_post.html" . -}}
+      {{- partial "post_canonical.html" . -}}
     </div>
     {{- end }}
   </header>