Add support for `nopush` attribute

[ebidel]

https://w3c.github.io/preload/#server-push-http-2

Link: </app/style.css>; rel=preload; as=style; nopush
Link: </app/script.js>; rel=preload; as=script

The above example indicates to an HTTP/2 push capable server that /app/style.css should not be pushed (e.g. the origin may have additional information indicating that it may already be in cache), while /app/script.js should be considered as a candidate for server push.