Enhancement: Consumer should be able to know committed position before starting to fetch

The current consumer seems to be lacking the ability retrieve the its position or its list of committed offset before it has started fetching data.

I.E. `position()` and `committed(timeout,cb)` seem to work only in the `on('data',...)` event handler

In the *Java* consumer, a call to
`KafkaConsumer.committed(TopicPartition)` can be made in the `onPartitionsAssigned` callback to `subscribe`
the trick there is that the Java client goes and fetches the committed offset if it hasn't got any local data.

It would be nice to have this feature in the js client