More meaningful title for Bilibili videos with multiple segments / pages

[Kimi-Arthur]

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Description of your issue, suggested solution and other information

Bilibili videos containing multiple segments / pages can have more meaningful video title.

For each individual video, currently ydl uses the title directly. For each segment other than the first one, it's actually the original title + "(x)", where x is the number of segment.

Say the title is "Cool video", the titles will be "Cool video", "Cool video(2)", "Cool video(3)".

However, a lot of multi-segment videos have meaningful segment names.

For example: http://www.bilibili.com/video/av1415480/

The titles can be:

1. 【经典】周杰伦全MV 【154P】 - 可爱女人
2. 【经典】周杰伦全MV 【154P】 - 完美主义
3. ...

[Kimi-Arthur]

Also the id can be improved too, like 1415480_1, 1415480_2 etc.