[fetch] expand Request.destination with more granular values (#211)

As per [discussion on IRC](http://logs.glob.uno/?c=freenode%23whatwg&s=2+Feb+2016&e=2+Feb+2016#c983589), I'm proposing that we break apart and expand the current "unknown" and "subresource" values for [#concept-request-destination](https://fetch.spec.whatwg.org/#concept-request-destination). This would allow us to address the Preload use case (see https://github.com/w3c/preload/issues/55) by remapping it to use `request.destination` instead of `request.initiator`, and I believe it's consistent with what we intended in https://github.com/whatwg/fetch/issues/93. 

/cc @sicking @ehsan @mikewest @yoavweiss
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/211

-- Commit Summary --

  * expand Request.destination

-- File Changes --

    M Overview.src.html (55)

-- Patch Links --

https://github.com/whatwg/fetch/pull/211.patch
https://github.com/whatwg/fetch/pull/211.diff

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