Stop effects if music stops playing #123

I really like that the effects are reactive to music. One thing that bugs me though, is that the effects still continue to show something once the music stops playing.

The solution could be a feature that dimms down any effect once it does not detect any beat for a given amount of time. The feature could be disabled by default and enabled with a global button in the right upper corner, so the default behavior does not change.

I would be willing to submit a PR, but first I would like to hear your regards or tips how it should be implemented so the PR gets accepted.