Feature Request: support for canonical links

[davidgs]

Describe the solution you'd like

Support for canonical links in front matter. I often post things to commercial sites, but also want to post them to my personal site later. In order for google and other search engines not to refuse to index those pages, I need to have a
<link rel="canonical" href="link-to-original-content />
on my post.

Describe alternatives you've considered

This is the simplest solution. I have been using it for years on earlier versions of torah, so I thought it might be time to contribute it back.

Additional context

[davidgs]

see PR #1001