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

Author: nikaera

layouts/_default/single.html

@@ -17,6 +17,7 @@ <h1 class="post-title">
       {{- partial "post_meta.html" . -}}
       {{- partial "translation_list.html" . -}}
       {{- partial "edit_post.html" . -}}
+      {{- partial "post_canonical.html" . -}}
     </div>
     {{- end }}
   </header>

layouts/partials/post_canonical.html

@@ -0,0 +1,9 @@
+{{ if and (.Params.canonicalURL) (.Params.ShowCanonicalLink ) -}}
+{{ $url := urls.Parse .Params.canonicalURL }}
+
+{{- if or .Params.author $.Site.Params.author (.Param "ShowReadingTime") (not .Date.IsZero) .IsTranslated (or .Params.editPost.URL .Site.Params.editPost.URL) }} | {{- end -}}
+<span>
+    {{- (.Site.Params.CanonicalLinkText | default .Params.CanonicalLinkText) | default "Originally published at" -}}
+    &nbsp;<a href="{{ trim .Params.canonicalURL " " }}" title="{{ trim .Params.canonicalURL " " }}" target="_blank" rel="noopener noreferrer">{{ $url.Host }}</a>
+</span>
+{{- end }}