[fetch] upload progress? (#21)

Is there any way I can attach a progress listener to the xhr.upload object used in fetch?
```js
xhr.upload.addEventListener('progress', function(){ ... })
```
since I use fetch to upload files (with FormData), I would find this very useful.

I initially reported this at https://github.com/github/fetch/issues/89, and was suggested to have it reported here


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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/21