Allow Flag to be set to enable tracking on email.

Postmark app has added [Open](http://blog.postmarkapp.com/post/87919491263/open-tracking-is-finally-here) tracking to see when emails have been opened. You enable this through a flag sent with your message.
This change allows you to set this flag with your configuration:

```php
class EMailConfig {
    public $postmark = array(
        'transport' => 'Postmark.Postmark',
        'uri' => 'http://api.postmarkapp.com/email',
        'key' => 'POSTMARK_KEY',
        'TrackOpens' => true
    );
}
```

This will enable tracking on all messages, Or, if you prefer, you can turn tracking on per message. Just call config before sending the message.

```php
$email->config(array('TrackOpens' => true));
```