Re: [fetch] Aborting a fetch (#27)

@WebReflection 

> How is that since nobody using Promises these days would pass a third callback + nobody believes it's possible to cancel them?

Consider:

```js
var getValue = (_ => {
  var p;

  return _ => {
    if (!p) {
      p = new Promise(r => setTimeout(r, 5000)).then(_ => "Hello world");
    }
    return p;
  }
}());

var p1 = getValue();
p1.cancel();

var p2 = getValue().then(v => console.log(v));
```

`p1` is cancelled, but that means `p2` is also cancelled. Without cancellation, `p2` would always successfully resolve. Introducing this change directly to `Promise` feels a bit dangerous.



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