text: Title: Support multithreaded profiling
  Body: 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.