Consider `KillStrategy`/`KillOptions` API for smarter process killing

[madelson]

• Should Kill() kill child processes?
• Should Kill() send signals?
• Configure the Kill() behavior for timeout/cancellation
• Consider changing the default strategy for timeout/cancellation

E. g:

Timeout(TimeSpan, KillStrategy?)
CancellationToken(TimeSpan, KillStrategy?)
Kill(KillStrategy?)