Webvtt-py

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* Support for saving cue identifiers

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The main goal of this release is a refactor of the WebVTT parser to be able to parse easier and give support to
new features of the format.

New features:

* Support for cue identifiers
* Support for parsing WebVTT captions with comments
* Support for parsing WebVTT captions with Style blocks
* Support for BOM in caption files
* Added method to write the captions to an opened file
* Convert WebVTT to SRT format
* Ignore empty captions in SRT format

Other:

* Refactored WebVTT parser

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The text for the caption is now returned clean (tags removed). The cue text could contain tags like:
* timestamp tags: *<00:19.000>*
* class tags: *<c.classname>text</c>*
* and others...
**Important**: It currently removes any tag present in the cue text. For example <b> would be removed.

Also a new attribute is available on captions to retrieve the text without cleaning tags: **raw_text**

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The goal of this release if to allow the WebVTT parser to be able to read caption files that contain metadata headers
that extend to more than one line.

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* Made hours in WebVTT parser optional as per specs.
* Added support to parse WebVTT files that contain metadata headers.

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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.