Add junit-style output

[roblinksdata]

I've noticed that we have recently started using then junit-style output from csvlint.rb inside our Jenkins pipelines.

It's helpful since it allows us to view any errors/successes in an easy to view location inside the Jenkins web UI.

We should add similar functionality onto the csvw-check tool so we don't lose this functionality.

[roblinksdata]

@josepajay just added this new issue.