add a _signal keyword to specify what signal will be used to kill the process on timeout

[davidvilla]

No description provided.

[amoffat]

on release-1.10 branch:

import sh
import signal

try:
    sh.sleep(3, _timeout=1, _timeout_signal=signal.SIGQUIT)
except sh.TimeoutException as e:
    assert e.exit_code == signal.SIGQUIT