[Feature Request] thread/multiprocessing traces

Omg first off, I just found this and I love this package!

I haven't looked into the source to see how feasible this would be, but it would be great to be able to visualize the stack traces for each running thread/subprocess. (I'm sure thread traces would be easier to handle than processes). This would be very useful in trying to debug where `thread.join` is hanging for example.