[gatsby-theme-minimal-blog]: Canonical URL support

[aleccool213]

Love the theme 👍 Hope to use it but I require this feature.

Summary

For most blogs, cross posting a piece of content is common. If you post a piece of content on multiple sources, search engines need a Canonical URL to be able to figure out what is the source of truth.

ref: https://www.shopify.com/partners/blog/canonical-urls

Basic example

In the front-matter of a post, add the canonical_url header which would add

Example:

---
title: A Better Way to use GraphQL Fragments in React
banner: ./images/photo1.jpg
date: "2020-05-11T22:12:03.284Z"
canonical_url: "https://medium.com/@yourboybigal/a-better-way-to-use-graphql-fragments-in-react-4f54bf862062"
description: Defining fragments inside the components that render data brings many benefits.
slug: "/better-way-to-use-graphql-in-react/"
---

Would result in this header tag being added to the blog post:

<link rel="canonical" href="https://medium.com/@yourboybigal/a-better-way-to-use-graphql-fragments-in-react-4f54bf862062">

Motivation

I cross-post my posts to dev.to and Medium and this is required for me to use this theme.

Note

I have implemented this a few times in other Gatsby sites so I dont mind tackling this.