`pubsub peers` subcommand

```
# list all pubsub peers 
> ipfs pubsub peers
<peerid>
<peerid>
...

# list all pubsub peers for a topic string
> ipfs pubsub peers [ <topic-string> | <topic-id> ]
<peerid-subscribed-to-topic>
<peerid-subscribed-to-topic>
...
```

from https://github.com/libp2p/pubsub-notes/pull/4/files too
