Feature: Update DNS Factory to accept complete Config object. Add new FallbackExecutor and use fallback DNS servers when Config lists multiple servers. (#179 and #180 by @clue)

// old (still supported)
$config = React\Dns\Config\Config::loadSystemConfigBlocking();
$server = $config->nameservers ? reset($config->nameservers) : '8.8.8.8';
$resolver = $factory->create($server, $loop);

// new
$config = React\Dns\Config\Config::loadSystemConfigBlocking();
if (!$config->nameservers) {
    $config->nameservers[] = '8.8.8.8';
}
$resolver = $factory->create($config, $loop);