feat: display author avatar images in blog post

[imfing]

Preview:

It will support

authors:
  - name: imfing
    link: https://github.com/imfing
    image: https://github.com/imfing.png
  - name: Octocat
    link: https://github.com/octocat
    image: https://github.com/octocat.png

and

authors:
  - foo
  - bar

• handle the case where no link is provided for an author
• support local images

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