Add '-' option to write report to stdout

[risto-liftoff]

Description:

When testing out a tool like gitleaks, I find it useful being able to send output to stdout. This PR adds - as an option to use for --report-path to write to stdout. It's a convention that - signifies stdout (or stdin depending on the context).

Checklist:

• Does your PR pass tests?
• Have you written new tests for your changes?
  No, I believe the existing tests in report_test.go provide sufficient coverage.
• Have you lint your code locally prior to submission?