docs: function waitForPort(port, options) {
    options = _.opts(options, {state: 'free', timeout: 30});
    let func = null;
  
    if (options.state === 'free') {
      func = () => isPortInUse(port);
    } else if (options.state === 'bound') {
      func = () => !isPortInUse(port);
    } else {
      throw new Error('Unknown value for option \'state\'.');
    }
  
    return retryWhile(func, {timeout: options.timeout});
  }