Support for custom canonical URLs

[ngarbezza]

Hi!

I'm adding AstroPaper to my personal blog and I came across the need for custom canonical URLs.

Looking at the code, I think that it can be added as an optional attribute to the posts' frontmatter and pass it forward through the Layout template.

What do you think? I'm happy to work on this improvement

[satnaing]

Sounds good!
You can contribute to this one if you wish.