Allow async rules

[robertklep]

For example, to match connections where the client doesn't send any handshake data (wait for a second, when a handshake was sent, don't match, otherwise do):

.addAsyncRule(dataPromise => {
  return new Promise((resolve, reject) => {
    setTimeout(resolve, 1000);
    dataPromise.then(reject);
  });
}, debugPort);