keep extra options (for other reporter)

other reporter may have more options, I suggest to keep extras options and pass them to the reporter

use case, with https://github.com/bnoguchi/node-notify-send, I can add timeout, priority and icon options. eg:

```javascript
var notifier = notify.withReporter(function(options, callback) {
    growl.icon(options.icon).notify(options.title, options.message);
    callback();
});
```