Add ES7 Observable Spec start event to Observer interface and trigger it

[benlesh]

It's an event that needs to be triggered synchronously when subscription starts.

Basically:

myObservable.subscribe({
  start(subscription) { console.log('subscription started'); }
});

Related #65 #266 #678

[trxcllnt]

@Blesh I think I have a local branch with this in it. If not, I'm still happy to take this on.

[benlesh]

I've already got the work underway, @trxcllnt... but I really appreciate the offer.

[benlesh]

Closing this because it looks like it's not going to happen in the spec, and it's not necessary for anything in this lib at the moment.

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.