planemo lint report (machine-readable)

I was looking into ways of integrating the rich `planemo lint` results into the Galaxy Tools VSCode extension in a similar way as [the planemo tests](https://github.com/galaxyproject/galaxy-language-server/pull/110).

I found in the `planemo lint --help` an option to generate a xUnit report with `--report_xunit`:
````
Usage: planemo lint [OPTIONS] TOOL_PATH

Check for common errors and best practices.

Options:
--report_level [all|warn|error]
--report_xunit PATH Output an XUnit report, useful for CI
testing

--fail_level [warn|error]
...
````
I tried to use the option but It seems the report file is not generated with the lint command. Anybody else experiencing this?

In case it is not working or there is an error in the `--help` documentation, I will be happy to implement a new option `planemo lint --report_json` or at least try to :)