[fetch] Add `preload` and `prerender` contexts (#36)

Current [list of contexts](https://fetch.spec.whatwg.org/#concept-request-context) contains `prefetch` but omits `prerender` and `preload`, which need to be added to the list. I'm thinking...

* `preload` -> `connect-src` (see https://github.com/w3c/preload/pull/17#issuecomment-89356310)
* `prefetch` -> no CSP policy? Is there a reason? Should it be `connect-src` as well?
* `prerender` -> intuitively, seems that it should be the same as `prefetch`.

/cc @yoavweiss @mikewest

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/36