Implemented WebVTT support:

- `convert` chooses SRT ↔ WebVTT from file extensions.
- `shift` and `stats` accept `.vtt`.
- Parser handles metadata, identifiers, settings, comments, STYLE/REGION blocks, and varied line endings.
- Added tests and README documentation, including metadata-loss limitations.

`python -m unittest discover -s tests` passes all 19 tests. `git diff --check` passes.