Add --sigterm flag

[liammcinroy]

Adds a --sigterm flag (matching coverage.py's sigterm flag) to still write the coverage output if a SIGTERM signal is sent.

Helpful for using slipcover in e.g. docker containers.

Tested with a simple while True: ... script and then a kill -SIGTERM. I'm not too familiar with what sort of cleanup may be necessary for more complex cases, so I'd welcome any suggestions on how to truly gracefully exit.

Also, this currently ignores --fail-under, since it exits immediately instead. I'm happy to update it to e.g. raise an exception that is caught by a context manager surrounding the runpy / exec calls, so that the normal flow in __main__ afterwards is maintained. I chose not to for now, since the atexit isn't currently doing that either.

Thanks for creating/maintaining this project!