Video related primitives should be provided in scraperlist

We are publishing more and more ZIM files with videos using many different scrapers.

Do do that we mainly:

• Re-encode videos/audio streams
• Handle sub-titles
• Display all of this using video.js

For the moment many of these pieces of video related functions are distributed in different places (in the scraper relying on them).

At least to me, this is:

• difficult to understand which scraper is at which status related to the development of these pieces of code
• Fixes will have to be requested/tracked in each scraper
• Releases are also de-facto made at very different times

I'm not prescriptive about the exact solution, but I believe we should try to consolidate this at one place.

まず、python-scraperlibを使用するscraper全体に現在分散している動画関連機能を洗い出します。これには、動画/音声の再エンコード、字幕、video.jsによる表示が含まれます。統合に向けた単一のアプローチを定義し、各scraperがそこから何を利用するのかを特定します。これらのプリミティブが単一の共有場所で保守およびリリースされれば完了です。