Add canonical link to page head

* If there is a `canonical` entry in the page front matter if it is
  present. Otherwise construct a URL to the current page. Further, strip
  out `index.html` so that the root URL doesn't include this as it's
  "canonical" location. Inspired by
  https://gist.github.com/bennylope/1894706
Add canonical locations for BNR mirrored posts
Merge pull request #1 from iamvery/canonical-links

Add canonical link to page head