feat: add canonical url support (#650)

When you want to publish content that is already published on a different site. You need to reference a canonical url of the original content.
By defining the `canonicalUrl` in the front matter definition the canonical url is set in the headers.

README.md

@@ -334,6 +334,14 @@ Dates of blog posts and single pages are rendered with the default date format c
 With hugo 0.87.0 and above, you can also use predefined layout, like `:date_full`, and it will output localized dates or times.
 See hugo's documentation of the [`time.Format` function](https://gohugo.io/functions/dateformat/) for more details.
 
+### Using a canonical url
+
+When you want to publish content that is already published on a different site. You need to reference a canonical url of the original content.
+By defining the `canonicalUrl` in the front matter definition the canonical url is set in the headers.
+
+```
+canonicalUrl: https://mydomain.com/path-to-the-oringinal-content/
+```
 
 ### Nearly finished
 

layouts/_default/baseof.html

@@ -31,6 +31,12 @@
       {{ end }}
     {{ end }}
 
+    {{ if .Params.canonicalUrl }}
+      <link rel="canonical" href="{{ .Params.canonicalUrl }}">
+    {{ else }}
+      <link rel="canonical" href="{{ .Permalink }}">
+    {{ end }}
+
     {{/* NOTE: These Hugo Internal Templates can be found starting at https://github.com/gohugoio/hugo/tree/master/tpl/tplimpl/embedded/templates */}}
     {{- template "_internal/opengraph.html" . -}}
     {{- template "_internal/schema.html" . -}}