Add XML output format to support CI test summary

[simonwilbert]

allow swagger-diff to output a file which CI systems can parse to get test summary

Circle - https://circleci.com/docs/test-metadata/

Like "Your build ran 832 tests with 4 breaking API changes and 3 Warnings listed below"

[zallek]

yeh that could be interesting. Currently, there are 2 output modes either "raw" or "json". We could add a "junit-xml"

https://github.com/windyroad/JUnit-Schema/blob/master/JUnit.xsd

So that's means that it needs to both output to stdout warnings/errors and also write to a file the junit test results.

that should be possible quite easily:

• update https://github.com/zallek/swagger-diff/blob/master/bin/swagger-diff.js#L30 and introduce a "junit-xml" outformat
• add a new reporter for junit next to https://github.com/zallek/swagger-diff/blob/master/src/reporter.js. the input of the report contains everything needed: lists of errors, warnings, info, etc. (cf current existing reporter).

Do you think you could create a PR for that it would be great :)