Add JUnit output format

[macbre]

• https://github.com/wesleyhales/loadreport/blob/master/loadreport.js#L481
• https://npmjs.org/package/jshint-junit-reporter
• http:https://stackoverflow.com/questions/4922867/junit-xml-format-specification-that-hudson-supports
• http:https://docs.casperjs.org/en/latest/testing.html#exporting-results-in-xunit-format

[rbayliss]

Is this something you're still looking for? It's certainly something I'd use. I could roll a PR unless this is a bad idea for some reason.

[macbre]

@rbayliss, if you feel like preparing a PR, that would be great :)