Add option for kill signal

[Øyvind Saltvik]

I need to send SIGINT and not SIGKILL, SIGTERM would be good too. On windows I am not sure if there is antyhing like this.

[Alexander Kachkaev]

I used yarn kill-port 3000 in CI but then realised it was not finishing the process the way I was expecting. A file that was supposed to be written on SIGINT was not there, so I switched to kill -2 $(lsof -t -i:3000) for now. This is Linux-only, but does the job in my case.

Having kill-port SIGINT 3000 would be great! List of signals: https://unix.stackexchange.com/a/317496