Allow Throttle to be piped to some time after instanciation

[vgire]

The following particular edge case does not work:

const throttle = new Throttle(1);
setTimeout(
  () => inputStream.pipe(throttle).pipe(outputStream),
  1000);

startTime is defined in constructor and not when inputStream is piped.
This PR fixes this issue.
Thank you for sharing your great work !

[jaredallard]

LGTM! 🎉