Make it possible to spawn commands with only a piped stdout

for `npm-exec` I want to pass spawned child processes `process.stdin` so that `process.stdin.isTTY == true` for the child process. However, bashful throws an error if the child process doesn't use `'pipe'` for both stdin and stdout. (It tries to call `cp.pipe(output, {end: false})` on line 433)