[Feature Request] Support for hardware volume control #109

It would be great if hardware volume control would be possible. This way sendspin could control the volume of any connected amp. In my case this is a hifiberry amp2. Just now it is possible that my sendspin client has volume 100% and the amp volume 0, so no music is playing. The sendspin client could use the hardware mixer from the soundcard, or from alsa, which does use hardware mixer by default, and change the amps volume instead of a virtual volume.

It is also notable, that the hardware mixers are optimised for their specific hardware and the connected dacs, which usually leads to better sound quality. The hardware mixer is also available for other soundcards, not only amps.