Implemented WebVTT support:

- `convert` selects SRT ↔ WebVTT from file extensions.
- `shift` and `stats` accept `.vtt`.
- Parser handles BOMs, metadata, optional identifiers, settings, short timestamps, and NOTE/STYLE/REGION blocks.
- README documents usage and metadata-loss limitations.

`python -m unittest discover -s tests` passes all 28 tests.