Stored message in event and used the event to get message during composing

@mtymek
Look at this:
https://github.com/juriansluiman/SlmMail/blob/master/docs/Mailgun.md

Message should not necessarily be instance of `Zend\Mail\Message`. For mailgun, it can be instance of `SlmMail\Mail\Message\Mailgun`. So , if we use mailgun, we can do something like this:
```php
$composer->getEventManager()->attach(ComposerEvent::EVENT_COMPOSE_PRE, function($event) {
    $event->setMessage(new \SlmMail\Mail\Message\Mailgun);
});
```