Joining the thread spawned by startProgress

[andersjel]

At the moment, it is hard to know when the thread spawned by startProgress is done
printing. This makes it difficult to put a newline after the last line and do any additional
output. Right now, one has to introduce an artificial delay or live with the race condition.

[roelvandijk]

Good point. It would be nice if we could solve this without needing an ugly hack like an artificial delay.

I'll have to think a bit more about the asynchronous interface.

[roelvandijk]

Implemented in 828817f. I used the async package which provides a good interface for this use case.