I'm trying to write some code blocks in Common-Lisp using GitHub Markdown on Atom, but if I write
it doesn't highlight the syntax.
Using the master list located in the github repo, we find common-lisp here: https://github.com/github/linguist/blob/master/lib/linguist/languages.yml#L743
So you need to use: ```common-lisp or ```lisp
for common-lisp in Github Markdown. If this does not show up in your Atom editor during the preview, then you need to install the language-lisp
package.