Enable easily publishing swing previews to GitHub Pages

[lostintangent]

While CodeSwing supports exporting your swings to CodePen, it would be ideal if there were a way to share a swing preview with others, without needing to rely on another service.

Since it's likely that many users will store/version their swings in a GitHub repo, we should explore how to make it easier to "publish" swings to a repo, and then host them via GitHub Pages.