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* Lots of improvements from PRs submitted by alucryd (big thanks!):
* Retain ASS styles;
* Support syncing several subs against the same ref via --overwrite-input flag;
* Add --apply-offset-seconds postprocess option to shift alignment by prespecified amount;
* Filter out metadata in subtitles when extracting speech;
* Add experimental --golden-section-search over framerate ratio (off by default);
* Try to improve sync by inferring framerate ratio based on relative duration of synced vs unsynced;