[feat] Publish source markdown

[rochacbruno]

when

marmite.yaml

extra:
  publish_md: true

Then for every render_html the original .md must also be published to the output dir,
and in the post page content.html there must be a link to the same slug but with .md extension
served directly.

If extra contains

extra:
  publish_md: true
  source_repository: https://github.com/rochacbruno/blog/content/

Then the link on content.html must point to the repo, so it is easy to get contributions and corrections on the text.