Add "--all" option to "core search" to show all available versions

[masci]

Fixes #183

Example usage:

~/work/arduino-cli(massi/corever) » ./arduino-cli core search mbed
ID           Version Name                            
arduino:mbed 1.1.2   Arduino nRF528x Boards (Mbed OS)

~/work/arduino-cli(massi/corever) » ./arduino-cli core search mbed --all
ID           Version Name                            
arduino:mbed 1.1.0   Arduino nRF528x Boards (Mbed OS)
arduino:mbed 1.1.1   Arduino nRF528x Boards (Mbed OS)
arduino:mbed 1.1.2   Arduino nRF528x Boards (Mbed OS)

[rsora]

LGTM