[5.5] Allow notification's template to be changed

[mateusjatenee]

Right now you can change the default markdown template either by changing the property or by creating another class that extends MailMessage and changing it there. This PR would allow something like this:

    public function toMail($notifiable)
    {
        return (new MailMessage)
            ->template('emails::foo')
            ->subject('Account Created')
            ->line('Welcome to foo app, bla bla bla')
            ->action('See Dashboard', url('/'))
            ->line('Thanks!');
    }

Not a new feature, but I think it looks way better this way.

Perhaps we could allow the default markdown template to be published and then the user could change it?