Creates multiple notifications

[webmink]

First, thanks so much for making this, I really like it! My apologies for not making a pull request but I'm not set up for development here (just a Chromebook...)

I notice you've left the notification ID empty in chrome.notifications.create, so every new track creates a new notification. That may be your desired behaviour, but if I'm reading the documentation right, if you set a value for notification ID the function will clear any previous notification when a new track plays.

[cheshire137]

That's a good idea. I'll look into this.