Allow collecting data for flame graph until interrupted

[christianglodt]

It would be useful if py-spy could be run in such a way that it would collect data for the flame graph until interrupted by a signal.

My use case would be to write a Django middleware that would attach py-spy at the start of a request, and interrupt it when the request is done, and then return the flame graph svg to the client instead of the actual content.
This would also allow to write a context manager that starts/stops py-spy during the run of a single block of code.

[benfred]

The last commit there should fix this

[christianglodt]

Excellent, I'll try it as soon as I can. Thanks!