Support loading GitHub gist files

[Kerollmops]

Hey 👋

Would it be possible to support loading the content from a GitHub gist like the Rust playground? Not necessarily being able to edit and save the content to a new gist or anything but simply being able to specify the gist we want to load when we load the page.

Have a nice day 🐑

[alvinhochun]

It seems that getting the contents of a gist is quite simple, doesn't require an API token and cross-origin seems allowed, so it is likely doable from client side JS. Overriding the initial code can be easily done in playground.vue, though we probably need a loading screen for it. If anyone is interested, feel free to implement it and make a PR.