[5.3] Add salutation option to SimpleMessage notification

[seblavoie]

By default 'Regards,' is displayed as a salutation at the end of notification emails. Much like the pull #15108 added an optional greeting() method, I think that would make sense to also add one for the salutation at the end. What do you think?

public function toMail($notifiable)
{
    return (new MailMessage)
                ->line('The introduction to the notification.')
                ->action('Notification Action', 'https://laravel.com')
                ->line('Thank you for using our application!');
                ->salutation("Cheers!")
}

[GrahamCampbell]

Probably should go to 5.4, at this point.

[taylorotwell]

I agree let's move this to 5.4 if you don't mind.