feat: Add option to report total coverage to stderr

[MisterMX]

This adds an optional flag -report-total-coverage-to-stderr that tells the converter to write the total coverage percentage to stderr.

This is usefull when working with CIs that rely on the logs to retrieve the total coverage for reporting, such as Gitlab CI.

In Gitlab CI this can be used in the following way:

go-test-report-coverage:
  scripts:
    - gocover-cobertura -report-total-coverage-to-stderr < go-test-coverage.txt > go-test-report-coverage.xml
  # Tell Gitlab to extract the coverage from the logs:
  coverage: '/^Total test coverage is (\d+.\d+%) of lines$/'
  # Regular coverage export for reporting (in addition to the total number above)
  artifacts:
    expire_in: 1 week
    reports:
      coverage_report:
        coverage_format: cobertura
        path: go-test-report-coverage.xml