Utilize canonical URLs if specified in front matter (#827)

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

### Prerequisites

Put an `x` into the box(es) that apply:

- [ ] This pull request fixes a bug.
- [x] This pull request adds a feature.
- [ ] This pull request introduces breaking change.

### Description

To prevent hurting the SEO when a blog post re-posted, it's recommended
to use canonical URL tags.
This PR updates the concerning metatag if `canonicalUrl` specified in
front matter.

See working example:
- https://kakkoyun.me/posts/fantastic-symbols-and-where-to-find-them
-
https://github.com/kakkoyun/me/blob/master/content/posts/fantastic-symbols-and-where-to-find-them.md

### Checklist

Put an `x` into the box(es) that apply:

#### General

- [x] Describe what changes are being made
- [x] Explain why and how the changes were necessary and implemented
respectively
- [ ] Reference issue with `#<ISSUE_NO>` if applicable

#### Resources

- [ ] If you have changed any SCSS code, run `make release` to
regenerate all CSS files

#### Contributors

- [x] Add yourself to `CONTRIBUTORS.md` if you aren't on it already

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Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>