Convert relative links to .html

[funnylookinhat]

When running a codo command that pulls in multiple .md files, it's nice to have them formatted as HTML, but the links within the markdown are no longer accurate. For example:

# My Readme

[Another Markdown File](path/to/SOME_FILE.md)

Correctly points to path/to/SOME_FILE.md in github and other markdown rendering services, but when pushed through codo the link should be updated to path/to/SOME_FILE.md.html if the reference exists.

[inossidabile]

This is interesting but not critical so I'm going to just leave it here not having time to work on that myself. PRs are welcome.