Let the severity level influence the json output

[alexanderameye]

When using yarn audit, the set severity level influences the json output when using the --json flag. It would be nice if this tool could as well have flags such as --critical influence the output when using --output-format 'json'. The --skip-dev flag influences it so it would make sense that the set severity level influences it as well.

[doricci]

This would be a great feature for one of my client 👍