Improve announce interface

[feross]

Currently setIntervals happen inside this package, but it has no way to know what upload/download numbers to report to the tracker.

Perhaps this should just be the responsibility of the user? There's already a client.update() function.

[yciabaud]

In my opinion we can allow the user to provide a callback to update the data when setInterval is triggered.

I can send you a PR in that way.

[feross]

Fixed by #107 and released as 7.3.0.