Template: unset rel=canonical and HTML5 meta charset

Use rel=canonical in accordance with [Google's guidelines](http://googlewebmastercentral.blogspot.com/2013/04/5-common-mistakes-with-relcanonical.html) (i.e. don't :smile:) and specify charset in HTML5-preferred format. Also rely on Python-builtin string formatting.

Did you consider submitting your plugin to [pelican-plugins](/getpelican/pelican-plugins)?