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This allows to show which option is selected with defaults or with
different FQBNs.

For example:

    $ arduino-cli board details arduino:avr:nano
    Board name: Arduino Nano

    Option:     Processor                         cpu
                ATmega328P                      ✔ cpu=atmega328
                ATmega328P (Old Bootloader)       cpu=atmega328old
                ATmega168                         cpu=atmega168

or:

    $ arduino-cli board details arduino:avr:nano:cpu=atmega328old
    Board name: Arduino Nano

    Option:     Processor                         cpu
                ATmega328P                        cpu=atmega328
                ATmega328P (Old Bootloader)     ✔ cpu=atmega328old
                ATmega168                         cpu=atmega168

See #45

There is no need for a fully fledged download library like `grab`.
This commit fix https://github.com/cavaliercoder/grab/issues/43#issuecomment-431076499

Fix #62

Fix #55

Fix #23

This commit fixes the text for the examples used when typing 'arduino-cli board attach' and 'arduino-cli board' as they shown the wrong commands (wrong AppName in the 'board attach' command, non-existing '--board' and '--sketch' arguments in 'board' command)

Fix #39

Fix #40

The conversion is required for GOOS=windows

Fix #31

See #6

Fix #6