Add junit output format for check results

[dmnk]

Creating an JUnit xml result of the check would make it easier to integrate sigma-cli into a CI/CD Pipeline and have a nice representation of the results / work on them.

Possible integrations (for possibly slightly different output formats):

• https://medium.com/@ctrf-io/github-actions-publish-and-view-test-results-report-23bf4f3872ff (they work on CTRF - Common Test Report JSON Files)
• https://docs.gitlab.com/ci/testing/unit_test_reports/ (working on JUnit XML result format ) - but the more I research the more I like the idea to (in the mid-term) integrate code quality results (utilizing the code climate report format)

use case:

• ensure a specific minimum level of quality of sigma rules in a repository
• have an overview of possible regressions when changing code