Add JUnit reporter

Extracted from #25 (comment 373866676) (assume #25 (closed) handles configuration setup):

For JUnit, easiest answer seems to be to follow the jest-junit model - create an appropriately configured JSON file and pass to the xml module (as seen at https://github.com/jest-community/jest-junit/blob/master/utils/buildJsonResults.js). To output results can save JSON file here.

The wrinkle is that while passed and failed are pretty straightforward, there really is no support for anything truly equivalent to warning. The spec support errors, but these are intended for tests not completing.

If warning is treated as passed or failed it means false positives or negatives, so may be best to assume one as a default, and give a configuration option to choose.