[permissions] Extensibility: make sure we have it right (#17)

The specification is meant to be extensible but the document is not very verbose about that mostly because the currently specified permissions do not require more than a name. I was actually close to remove ```Permission``` altogether.

We should test the extensibility of the specification with some permissions that are currently available on the Web but not in the specification for now. I will create a separate document in the repository for that so it's clearer that we have some cases in mind without polluting the specification with things that no one is intending to implement in the short term future.

CC @marcoscaceres @slightlyoff @jakearchibald

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/permissions/issues/17