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.

首先，盘点目前分散在使用 python-scraperlib 的各个 scraper 中的视频相关功能，包括视频/音频重新编码、字幕和使用 video.js 的显示。定义一种统一方案，并确定每个 scraper 将从中使用哪些内容。当这些原语由一个共享位置统一维护和发布时，即视为完成。