GitHub CSS is out of date

[hraftery]

MacDown bundles GitHub.css and GitHub2.css for use as rendering options. Both were last updated 4 years ago and seem to be jarringly different from the current rendering on GitHub.

I see Tools/GitHub-style-generator exists in this repo and appears to be designed to use the rather official looking primer-markdown (now superseded by primer-css) to derive the current GitHub markdown css. However, it does not seem to have been used to update what gets bundled with the app lately.

In the meantime, I used sindresorhus/github-markdown-css to manually produce a suitable CSS file that is working well for me. I've documented the method I used here. One clarification - I got the padding properties from GitHub2.css.

While this file solves the issue for me for now (and may help others), it probably does not belong in the repo because it's not autogenerated. I feel like either Tools/GitHub-style-generator or sindresorhus/generate-github-markdown-css would be a better source.