Support multithreaded profiling

Thanks for pyintrument - it's incredibly useful

I had need to trace a multithreaded python app and examine the relationship between threads. Obviously in some cases multithreading can be a little interesting in python, but in this particular case works well.

I have extended a fork of pyinstrument to support showing all child threads from the one where profiling starts. I get nice results with all threads separated which has been hugely helpful.

I'll file a PR and reference this issue. It's still a bit of a WIP, but i'd be curious to see if it looks reasonable to you.