[whatwg/fetch] Add hooks for controlling `dns-prefetch` and `preconnect` (#683)

As outlined in https://github.com/w3c/webappsec-csp/issues/282#issuecomment-357194535 we need to add better controls for `dns-prefetch` and `preconnect`, which will require some Fetch hooks. Opening this issue to track the related Fetch changes.

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https://github.com/whatwg/fetch/issues/683