Support consumer media keys.

[willwade]

This is needed for iOS etc control. See AT+BLEHIDCONTROLKEY at https://learn.adafruit.com/introducing-adafruit-ble-bluetooth-low-energy-friend/ble-services and consumerKey here https://learn.adafruit.com/introducing-the-adafruit-nrf52840-feather/blehidadafruit

[RoganDawes]

Still trying to convince you :-)

VNC would make this simple, as you just need to detect the local key event, and make sure that there is a keyboard mapping for it on the bluetooth side.

[willwade]

Initially done with #129 - VOLUP VOLDOWN or MUTE. adding a whole new AT command avoided but if this needs it we can review.