Volume control, second-tier listening, tune-in functionality, and silence-based timeout of voice control.

[maso27]

• Voice control via CMU Sphinx - listens until silence is detected
• Volume control
• Second-tier listening - sounds a beep and waits for user response when Alexa asks for more information
• TuneIn works better
• Pressing the button for 10 seconds triggers computer shutdown
• Option to install Shairport-sync for Airplay support
• Option to enable always-on monitoring in case of crashes

The TuneIn, volume control, and some silence detection was thanks to nascentobjects. The initial CMU Sphinx implementation was thanks to fcooper.

[sammachin]

I've finally had some time to look at this project, I've spent the last week refactoring the code to be a lot more modular so that we can support multiple different setups and different ways of triggering eg, button, Spinx, Snowboy etc. I'm going to close this PR but will try and incorporate but where I can then hopefully we can move forward on the new modular codebase.