Add GitHub Action CI

[kianmeng]

This resolves #43.

[josevalim]

💚 💙 💜 💛 ❤️

[kianmeng]

🎉 🎉 🎉 🎉 🎉

[kianmeng]

I've noticed you've disabled the mix format job. Should we add the .formatter.exs and mix format the whole project?

[josevalim]

I will let @tmbb do it if he wants to. otherwise we can do it later :)

[tmbb]

I will let @tmbb do it if he wants to. otherwise we can do it later :)

I believe there was once a reason we weren't blindly running the formatter, right? Maybe something to do with the modules that are generated from Pygments? I can't recall what the reason was, and if no one can either, we could run the formatter.