Use default FQBN options if not specified

For example in #17 and https://github.com/arduino/arduino-cli/issues/23 is not immediately clear that

`arduino:avr:mega` requires the extra option: `arduino:avr:mega:cpu=atmega2560`

At least we should allow to write `arduino:avr:mega` for the default settings.
