I would like to modify atom.io's markdown syntax specification for some special purpose editing environment (academic texts), but I fail to identify the language file related to Github Markdown among atom.io's packages. It looks like every single syntax, except Markdown, has a corresponding language file, including plain text! Maybe I am looking in the wrong repository. It will be a great if you can point me to the right one.
Have you had a look at the https://github.com/atom/language-gfm repo? It's the GitHub Flavored Markdown language.
That might be what you're looking for, as you're right, there does not seem to be a dedicated (non-GitHub) Markdown support.