This is an attempt to fix the problem described by the issue sindresorhus#96.
If a child process is killed, the descendents of that child process won't be killed as well. This happens on Linux but not on Windows [^1].
The adopted solution is the "PID range hack" [^2] that uses the `detached` mode for spawning a process and then kills that child process by killing the PID group, using `process.kill(-pid)`, effectively killing all the descendents.

*Implementation*

- added an internal option `killByPid` as a remained for the spawned child process that it will be `detached` and to kill it by PID
- expanded and moved to a separate function the routine to kill the spawned process to `killSpawned`
- the `ChildProcess#kill` method of the spawned child process will be replaced by the `killSpawned` routine, to kill by pid if necessary
- the `killSpawned` routine signals that the child process has been killed, if it has been killed by the pid

I checked and all the tests pass.

This implementation also considers the issue sindresorhus#115 and shouldn't interfere with the detached/cleanup fix.

[^1]: https://nodejs.org/api/child_process.html#child_process_subprocess_kill_signal
[^2]: https://azimi.me/2014/12/31/kill-child_process-node-js.html