--format json results in no output

[djmcgill]

Hi, I'm trying to write a wrapper around this cli so that I can call it from build scripts: https://github.com/djmcgill/arduino-cli-rs
However, --format json currently doesn't seem to output any text at all. I've tried a few of the commands - core search and core list for example. As shown below --format text works as expected.
If this is just a case of not-yet-implemented is there a roadmap or issue board I can follow?

~/code/arduino-cli-rs master*
> arduino-cli core list --format text
ID              Installed       Latest  Name
arduino:samd    1.6.19          1.6.19  Arduino SAMD Boards (32-bits ARM Cortex-M0+)

~/code/arduino-cli-rs master*
> arduino-cli core list --format json

~/code/arduino-cli-rs master*
> echo $?
0

[cmaglie]

I've added a json output with a minimum amount of information on #48, we could add more as needed.