Add possibility to get all peers of a pubsub topic to js-ipfs-api

[haadcode]

We have a way to get a list of peers that we're pubsubbing to but it's not exposed in js-ipfs-api currently.

This would be hugely beneficial for Orbit, and others too, so let's add that.

https://github.com/ipfs/go-ipfs/blob/master/core/commands/pubsub.go#L267

[victorb]

Ref for pending PR adding pubsub (but not this command, would be after): #377

[daviddias]

If I understand correctly, it is done here: #471