Support preload/as

So, my app:

* Makes an XHR request for an HTML document
* Replaces current `body` with that document's `body`

Using `rel=prefetch`, browsers don't want to use the prefetched document and instead make a new request. Using `rel=preload`, though, all browsers (FF nightly, not current build) correctly make no new network requests and get the XHR requested doc from the cache.

I accomplish this right now with a super hacky fork:

Is this usecase legitimate? Should it be an option on quicklink? https://github.com/speedshop/quicklink/commit/204723d8f812000a8d73786fd86b598e6154b864