[BitChutePlaylist] Add new extractor

[sidneys]

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• At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

---

Description of your pull request and other information

This pull request adds the BitChutePlaylistextractor, introducing playlist download support for BitChute.

[ghost]

Your PR seems to need a fix to eliminate duplicate entries.

With this playlist https://www.bitchute.com/playlist/9UsDwfa6PzoQ/, 22 videos are extracted, whereas it actually contains 19 videos.
In the output below, last 3 entries are duplicated:

$ python -m youtube_dl https://www.bitchute.com/playlist/9UsDwfa6PzoQ/ --flat-playlist -j
{"url": "https://www.bitchute.com/video/Mil3YhjHEL8Z", "_type": "url", "ie_key": "BitChute", "id": "Mil3YhjHEL8Z"}
{"url": "https://www.bitchute.com/video/FeWkrZGVZovZ", "_type": "url", "ie_key": "BitChute", "id": "FeWkrZGVZovZ"}
{"url": "https://www.bitchute.com/video/BJkbSBOcopou", "_type": "url", "ie_key": "BitChute", "id": "BJkbSBOcopou"}
{"url": "https://www.bitchute.com/video/IdbWp32BZoTC", "_type": "url", "ie_key": "BitChute", "id": "IdbWp32BZoTC"}
{"url": "https://www.bitchute.com/video/rKM7Mo3myFGc", "_type": "url", "ie_key": "BitChute", "id": "rKM7Mo3myFGc"}
{"url": "https://www.bitchute.com/video/kcMmN8ejkY8s", "_type": "url", "ie_key": "BitChute", "id": "kcMmN8ejkY8s"}
{"url": "https://www.bitchute.com/video/tTXG8cNnQoA", "_type": "url", "ie_key": "BitChute", "id": "tTXG8cNnQoA"}
{"url": "https://www.bitchute.com/video/_4sH9cki1KQ", "_type": "url", "ie_key": "BitChute", "id": "_4sH9cki1KQ"}
{"url": "https://www.bitchute.com/video/f6Y8dt_Mmqw", "_type": "url", "ie_key": "BitChute", "id": "f6Y8dt_Mmqw"}
{"url": "https://www.bitchute.com/video/oMTZwAZVwGQ", "_type": "url", "ie_key": "BitChute", "id": "oMTZwAZVwGQ"}
{"url": "https://www.bitchute.com/video/zmtjJ2rC5gmS", "_type": "url", "ie_key": "BitChute", "id": "zmtjJ2rC5gmS"}
{"url": "https://www.bitchute.com/video/4doTXt_Ink8", "_type": "url", "ie_key": "BitChute", "id": "4doTXt_Ink8"}
{"url": "https://www.bitchute.com/video/HXT4EqJIzaM", "_type": "url", "ie_key": "BitChute", "id": "HXT4EqJIzaM"}
{"url": "https://www.bitchute.com/video/_-rpR7M9pEM", "_type": "url", "ie_key": "BitChute", "id": "_-rpR7M9pEM"}
{"url": "https://www.bitchute.com/video/agZRBKWvO1g", "_type": "url", "ie_key": "BitChute", "id": "agZRBKWvO1g"}
{"url": "https://www.bitchute.com/video/WVDFj9MutYo", "_type": "url", "ie_key": "BitChute", "id": "WVDFj9MutYo"}
{"url": "https://www.bitchute.com/video/aCuSC19Lxjtr", "_type": "url", "ie_key": "BitChute", "id": "aCuSC19Lxjtr"}
{"url": "https://www.bitchute.com/video/plRpBpwBE7hi", "_type": "url", "ie_key": "BitChute", "id": "plRpBpwBE7hi"}
{"url": "https://www.bitchute.com/video/wt6MCBkwMKzj", "_type": "url", "ie_key": "BitChute", "id": "wt6MCBkwMKzj"}
{"url": "https://www.bitchute.com/video/plRpBpwBE7hi", "_type": "url", "ie_key": "BitChute", "id": "plRpBpwBE7hi"}
{"url": "https://www.bitchute.com/video/wt6MCBkwMKzj", "_type": "url", "ie_key": "BitChute", "id": "wt6MCBkwMKzj"}
{"url": "https://www.bitchute.com/video/wt6MCBkwMKzj", "_type": "url", "ie_key": "BitChute", "id": "wt6MCBkwMKzj"}

[Cyberes]

I'm not seeing any duplicates in my tests. I piped kikuyan's command to uniq --count and there weren't any duplicates.

[ghost]

I tried again with 2021.06.06 plus this PR, git master plus this PR, and got the same results as before. Strange.