Add support for setting custom Request options.

To use custom timeouts, proxies, and other Request features,
call `SmartFilter#setOption()` or `SmartFilter#setOptions()`:

    $apiKey = 'your api key';
    $filter = new SmartFilter($apiKey);

    // Override custom timeout of 10s
    $filter->setOption('timeout', 3);

    // Use a proxy
    $filter->setOption('proxy', 'http://myproxy');

    // Or set many options at once
    $filter->setOptions(array(
      'timeout' => 3,
      'proxy' => 'http://myproxy'
    ));

    // Now all your API calls will use the options you passed in
    $filter->filter($userInput); // uses proxy and custom timeout