[Pitch] SIGINFO (et al.) support

Hi folks! Something you see in a lot of command-line tools is the ability to request information from a running process via a platform-specific mechanism: On Apple platforms, FreeBSD, and OpenBSD, by sending SIGINFO to the process or by pressing Ctrl+T in the Terminal application. On Linux, by sending the SIGUSR1 signal to the process. On Windows, by pressing Ctrl+Break in the Terminal application. I've been working on implementing support for this functionality in Swift Argument Parser here....