Mute audio when Moonlight not focused

[CyrisXD]

I'll often switch back and forth between games and or work. It would be good to be able to have the Moonlight window mute when it isn't focused.

For example, if you're having a game session and you get a Skype call through. You'd have to completely close Moonlight to stop the stream and stop the audio playing so you can answer the skype call.

I'm on MacOS using Moonlight-Chromem, which work great by the way! Great work on this.