Format for 'wp core update' #129

Feature Request

• Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

wp theme update and wp plugin update support the --format parameter, but wp core update does not, while I need JSON output.

Describe the solution you'd like

I'd like the --format parameter to be added to wp core update, unless there is a specific reason it is available for both theme and plugin and not for core...