Platformio supports multiple environments and sharing a global configuration.
Make use of this to easily allow switching between the production build, with HID,
and the development build, with serial.
Update the README with information on how to build those environments.
Implement a set of functions to create messages following the protocols
defined in the README for communication.
Handle HID, which is used to communicate with the interactive application.
Update README with details on the message structures.
