Add option to output canonical URL

Canonical links are handy to tell search engines which page is
canonical amongst various permutations of URLs (e.g. https vs. http
and www subdomain or not).  Add an option to output one via the
following pelicanconf.py setting:

REL_CANONICAL = True

templates/base.html

@@ -115,6 +115,9 @@
     <meta name="msapplication-TileColor" content="{{ BROWSER_COLOR }}">
   {% endif %}
 
+  {% if REL_CANONICAL %}
+    <link rel="canonical" href="{{ SITEURL }}/{{ output_file }}">
+  {% endif %}
   {% block meta %}
     <meta name="author" content="{{ AUTHOR }}" />
     <meta name="description" content="{{ SITEDESCRIPTION }}" />