[5.3] Add support for setting mail notification priority

[KKSzymanowski]

Usage:

class Foo extends Notification
{
    public function toMail($notifiable)
    {
        return (new MailMessage)
            ->priority(1);
    }
}

Priority levels like in Swift message: 1 - Highest, 3 - Default, 5 - Lowest.
It would be nice if we would have some constants for this. Not sure if it belongs here or on Swift\Message. What do you think?