[5.6] Allow passing of recipient name in Mail notifications

[Propaganistas]

MailChannel only passes an email address to the Mailer when sending mail notifications. It's a common situation to also know the notifiable's name and hence it seems appropriate to include this as the recipient's name in order to gain trust:

hello@example.org
   vs
John Doe <hello@example.org>

This rather small fix allows mail notifications to include the recipient name by using common syntax in the routeNotificationForMail() method:

/**
 * Route notifications for the Mail channel.
 *
 * @param  \Illuminate\Notifications\Notification  $notification
 * @return string
 */
public function routeNotificationForMail($notification)
{
    return [
        $this->email => $this->name,
    ];
}

Multiple addresses remain supported, with or without a set name:

/**
 * Route notifications for the Mail channel.
 *
 * @param  \Illuminate\Notifications\Notification  $notification
 * @return string
 */
public function routeNotificationForMail($notification)
{
    return [
        $this->email => $this->name,
        $this->email_2,
        $this->email_3 => $this->otherName,
    ];
}