Add Jinja template for Protobuf message types

Add I2C forwarding by default.  I2C forwarding can be disabled by one of
the following:

 - Pass `disable_i2c=True` when rendering the Procotol Buffer
   definitions jinja2 template.
 - Define `DISABLE_I2C` in the Arduino source code _before_ including
   the `CommandProcessor.h` header.