Add `junit` support

[rn]

junit is a "standard" XML based file format to report unit test results and there seems to be some tooling support for it as well (at least in Jenkins). Would be good to add junit support to rtf to take advantage of this tooling and to allow for better integration with Jenkins based CI systems.

Here is a link to some documentation I found: http://llg.cubic.org/docs/junit/ and here is a link to the XSD schema: https://github.com/windyroad/JUnit-Schema

[dnephin]

https://github.com/jstemmer/go-junit-report/blob/master/formatter/formatter.go has some Go types for this format.

[rn]

Thanks, good to know