[telegram] Add new extractor

[ESWZY]

Please follow the guide below

• You will be asked some questions, please read them carefully and answer honestly
• Put an x into all the boxes [ ] relevant to your pull request (like that [x])
• Use Preview tab to see how your pull request will actually look like

---

Before submitting a pull request make sure you have:

• At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
• Searched the bugtracker for similar pull requests
• Checked the code with flake8

In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

---

Description

Add new extractor for Telegram public channels like https://t.me/s/telegram/86

[dstftw]

Covered by generic extractor.

[ESWZY]

Covered by generic extractor.

Hi Sergey,

Thanks for your review.

The difference between this extractor and generic extractor is that this extrator can download video in specific message.

But, the generic extractor will download all of videos in this page, not the video in this message.

For example, if I want to download video in 86th message, the generic extractor will download two video 76, two video 80, two video 82, video 86, video 90. That may be difficult to manage.

So, I want to keep the extractor. Hope you can consider that.

Thanks.