fixes an issue on MacOS Big Sur 11.6 (#288)

[johnlee999]

After upgrading MacOS to 11.6, terminal-notifier shows another notification with a message "terminal" when you click the original notification. (see issue #288 )
It seems that terminal-notifier can't receive any message by notification center from 11.6.
So it makes 'userNotification' always null.

Thus, this never runs.

 if (userNotification) {
    [self userActivatedNotification:userNotification];

Terminal-notifier quits after it delivered a notification.
But if terminal-notifier is still alive after the notification, it can receive the message.
So I tried to make it keep alive as a daemon.

I'm not sure yet if there's the side effects of allowing it to be resident, but it can now opens url I gave by clicking the notification.

[synconclick]

Hello, can you please provide the steps on how you kept Terminal-notifier alive as a daemon? Thanks

[johnlee999]

@synconclick
Hi.
Basically, it's controlled by isDaemon flag. Could you check the diff of my commit?