Support AdVent shield for Rasberry Pi

[denis-stepanov]

By design, AdVent cannot, and probably will never work 100% error free. When it makes mistakes, usually some kind of manual action on console or on TV control is needed. To facilitate its use without an external terminal, it would be good to support a simple shield for Pi consisting of these elements:

1. a two-position switch turning recognition (and the associated TV actions) on and off. The use would be to deactivate AdVent without unplugging the Pi when its use is undesired;
2. a push button emulating a hit on short press or some other action on long press. The use would be to correct recognition mistakes, and, potentially, save audio around the event when asked (see issue #32);
3. an RGB-LED to signal current status (active or not; missing input, hits, etc).

If possible, it would be also good to avoid modifying an already present HiFiBerry shield when making such a shield, as soldering anything to HiFiBerry in general voids its warranty.

[denis-stepanov]

Following v1.6 features, other three buttons could be considered to emulate TV mute and volume up/down buttons.