[push-api] Expose PushSubscriptionOptions value on PushSubscription (#184)

We should expose the options that were set in a call to `subscribe()`.

Two concerns: 
* we probably *don't* want to serialize these.
* we need to make it clear that unsupported attributes will not be saved (setting options to `{ chocolate: false }` will not result in a value being saved for you. 

As mentioned in #182 and #183.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/184