Re: [fetch] Mechanism to indicate "destination context" (#64)

@annevk the difference between "navigate to next" and "used by current page" is already captured via the source context (i.e. `prefetch` vs `preload`). 

`type` is one common interface that can be communicated both by the developer and the server to the UA (see [proxy/server initiated fetching use case](http://w3c.github.io/preload/#developer-server-and-proxy-initiated-fetching)): the developer can declare what type they're fetching, proxies can observe fetched resources and communicate same information to the UA (e.g. `<https://example.com/logo-hires.jpg>; rel=preload; type=image/jpeg`). The UA logic is as outlined in https://github.com/whatwg/fetch/issues/64#issuecomment-115985753.

That's not to say that we might not want to expose additional context to the UA to further improve prioritization logic, but I do think `type` is necessary to solve some of the existing prefetch/preload use cases.

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