Add MailcoachMessage class for use in notification classes

[gdebrauwer]

When you want to sent a mail using mailcoach from within a notification class, you have to create a mailable class and call that class from within your notification

class FooNotification extends Notification
{
    public function via()
    {
        return ['mail'];
    }

    public function toMail()
    {
        return new FooMail();
    }
}

Having to create a mailable class is a bit annoying. This PR fixes that by allowing you to return a mailcoach message right from within your notification classs.

class FooNotification extends Notification
{
    public function via()
    {
        return ['mail'];
    }

    public function toMail()
    {
        return (new MailcoachMessage())
			->usingMail('mail-template-name')
			->replacing('some-key', 'some-value');
    }
}

[freekmurze]

Might be good to add an example of this in the readme as well.

[Nielsvanpach]

Thanks for your work on this!