reff - Rename files in a directory to date + index + original extension.

Usage:
  reff <directory>

Example:
  reff ./photos

  If the directory has 150 files, they will be renamed to:
  2026020600001.<ext>, 2026020600002.<ext>, ... 2026020600150.<ext>
  (date is current date YYYYMMDD, index is 1-based zero-padded to the width of total count)

Build:
  cargo build --release
