Detect & filter silence

[MarkKremer]

It would be nice to detect remove silence from an audio stream. See #112 for the original request.

In my early Beep days I've also played with this. It's possible to look at the root mean square of individual samples over a period of time and average that. However, that doesn't give a good indication of the perceived loudness.

EBU_R_128 may be a better way to detect perceived loudness. Although I don't know how difficult it is to implement and how fast it is.

I would accept PR's for any of these implementations.