New

• Retain selected elements' original values using --keep <tag_name>.
  The argument may be repeated to retain multiple elements' values.
  Elements are identified by name (e.g. "PatientName", "StudyDate") or by
  hexadecimal tag value (e.g. "0010,0010"). (#155)
• Anonymizer.add_element_handler allows addition of new element handlers (even custom ones)
  to override existing behavior. (#155)
• New element handler type dicognito.value_keeper.ValueKeeper that will
  keep element values as-is. (#155)
• New --what-if flag shows intended anonymization actions (as appear in the project README)
  (#163)

Fixed

• --output-directory may create duplicate anonymized files (#161)

Additional Items

• Format code using Ruff (#156)