There should be an async `flush()` method.

[aswyer]

While #28 would solve my specific issue, I think it makes sense to provide a flush() method that is async so I can perform something only after the events have been sent. There is currently no way to know when the events have been sent.

[aswyer]

I was going to post a PR for this, but I don't seem to have permissions. Attached is a patch: fix.patch.

Update: was able to post a PR here.