Fix notification sound not playing

[notlmn]

Fixes #272

[jeesim2]

@notlmn
Today I spent 2 hours to figure out what apps started to play sound on my macbook from yesterday,
and finally check this plugin that totally forgotten in my mind.
(the sound played very often, once~several times a minute even though there is no new notifications on github)

After I installed this plugin a year ago there were no sound notification.
Because It was such a long time ago, today I couldn't consider this plugin as the sound source.

I guess many users who installed this plugin long time ago might fall in problem to find the sound source.

[notlmn]

If you think there is a bug with the behavior documented in readme, please open a new issue.

Otherwise this is a case of https://xkcd.com/1172

The option is disabled by default for users, unless you have had it enabled before or enabled now, you should not be hearing any sound.