Make the normalized markdown work with github markdown, if possible

Given the constraint of #339, make sure the normalized markdown format also gives the same HTML result with the github markdown parser, if possible.

Github seems to use one of these:
- https://github.com/vmg/sundown
- https://github.com/github/cmark
- https://github.com/github/markup

and commits to following CommonMark, with strictly optional extensions.

Start by reading the constraint in issue #339, then review the linked sundown, cmark, and github/markup projects to determine which GitHub Markdown parser is relevant. Compare the normalized Markdown's rendered HTML with GitHub's result; done means the outputs match where the parser supports the same constructs.

Written by the indexing model from the issue text.