pmctl: add a control CLI to pm and supervisor

@sam-github @kraman I merged #14 too hastily, it seems. I've reverted it, re-branched, and cherry-picked the original commit.
This blocks the event loop, which means instead of seeing the familiar pattern of a strong-supervisor restart loop due to a missing module through the strong-pm server output and .pipe()'d to process.stdout, we just get a silent test timeout.