Sync-subtitle

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* Added sync_after_index()
* Added sync_before_index()
* Added sync_between_indexes()
* Changed 'input_srt_file' argument to 'input_file'
* Changed 'output_srt_file' argument to 'output_file'
* Fixed delay and hasten logic
* Refactoring

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* Changed sync_subtitle() to sync()
* Added sync_after_time()
* Added sync_before_time()
* Added sync_between_times()
* Added support to optionally specify an output filename.
In absence of an output filename, in-place modification will be done.
* Input file extension check is added
* Edited and formated the docstring
* Releasing under MIT license

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* Initial release with sync_subtitle()