text: Title: --update-snapshot without overwriting unused ones
  Body: **Is your feature request related to a problem? Please describe.**
  Working with a large test suites we have some tests that are being skipped depending on environment. When writing new tests with snapshots and then running `--update-snapshot` this overwrites the skipped snapshots which requires reverting the changes.
  
  **Describe the solution you'd like**
  A flag `--skip-unused` that skips the unused snapshots which would also hide the error/warning about unused snapshots.
  
  **Describe alternatives you've considered**
  Reverting the changed snapshots manually
  