add a self-referencing canonical tag

Author: lausser

layouts/partials/head.html

@@ -1,5 +1,6 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
+<link rel="canonical" href="{{ .Permalink }}" />
 {{ range .AlternativeOutputFormats -}}
 <link rel="{{ .Rel }}" type="{{ .MediaType.Type }}" href="{{ .Permalink | safeURL }}">
 {{ end -}}