Support CORS requests from the documentation and the HOMEPAGE zomg

[shepmaster]

Looks like the documentation does some in-page XHR and gets the result.

• Go to https://doc.rust-lang.org/stable/book/first-edition/variable-bindings.html
• Click on the "play" icon in a code sample

It makes an OPTIONS request to https://play.rust-lang.org/evaluate.json. Will need to investigate the exact schema.