Update GitHub styles from current Primer build

[jsejcksn]

There have been some tweaks to the Primer Markdown styles since 3 years ago, and it looks like the file that is responsible for those styles in this package is primer-markdown.less

Can we get this updated so that the setting

• Use GitHub.com style

actually reflects the current GitHub.com style?

I'm not sure if it's being generated by a script or if it needs to be manually updated.

[lee-dohm]

Maintaining this would be easier if the preview was in a webview element or some such that would isolate the CSS from the rest of Atom's UI.