Webvtt-py

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New features:

* Added support for YouTube SBV captions.
* Added easy iteration to WebVTT class.
* New CLI command for segmenting captions for HLS.

Other:

* Improved parsers to reuse functionality.
* Added an exception for invalid timestamps in captions.
* Added an exception when saving without a filename.

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* Refactor of the main module and parsers.

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This module is released with the following initial features:

* Read/Edit/Write WebVTT captions.
* Read SRT captions and convert to WebVTT.
* Segment WebVTT files for captioning HLS video.