Xo: Metadata is missing from reports

[GriffinSauce]

I'm writing a formatter based on the ESLint spec only to discover that the metadata is not returned from XO.
That blocks quite a lot of value.

In mergeReports the metadata should be merged as well and then passed as the second parameter in log.

Background:
https://eslint.org/docs/developer-guide/working-with-custom-formatters#the-data-argument

This currently already breaks the json-with-metadata reporter.

[sindresorhus]

I agree. It's not something I plan to work on, but a pull request is welcome.