[permissions] Add getUserMedia permissions (#10)

I think getUserMedia has a minimum of two permissions ```capture-audio``` and ``capture-video```. But we could try to merge them into one like:
```js { "name": "capture", "audio": true, "video": true }```

I'm not convinced that having the object would be better but I think we should get some feedback from the spec editors. IIRC, @marcoscaceres, we tried to do that and it didn't work. Do you remember what hoppened?

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/permissions/issues/10