Markdown output format

[wendorf]

Thanks for yarn-lock-diff! It's really valuable.

I like to use it when building a PR message, especially when the PR is just version bumps for my yarn.lock. To display nicely on GitHub (and other places that use Markdown), I need to do some string editing before I can paste. Not a big deal, but I'd love to be able to do something like bin/run --old yarn.old.lock --new yarn.new.lock --format markdown.