[TASVideos] Add new extractor

[EdenOvrutskiy]

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• Searched the bugtracker for similar pull requests
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• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

tasvideos.org is a website that hosts videos of tool-assisted speedruns in videogames.

According to it's legal notice :
"Movies published on this site were created by and for this site."

To watch a video on the site, click "watch now" below the thumbnail of an entry to display a video-player.
Alternatively, the site offers direct download links for various formats on each entry.
Example: http://tasvideos.org/4352M.html

"Playlists" are search results via the filters on the website. Each filter adds something to the URL,
but all URLs always starts with "tasvideos.org/Movies-.."
Example: http://tasvideos.org/Movies-Stars.html

I tried making this extractor as minimal as possible, to see if I got the hang of things.

Also - I had no choice but to add video_url = "http://www." + self._search_regex( ...
Because the website's links don't contain the http prefix.
'M' is added to playlist video ID's to match the single video url id's format
(getting the id + 'M" suffix + download links in one regex proved too difficult)

answers - #26765

Since the addition of timer + credit, I'm trying to change the title field to:
'title': 'C64 L'Abbaye des Mortsin 02:36.7 by Mothrayas.',
(at _TEST)
But github doesn't let me.
Would like to avoid making a new PR, but I don't have the folder with the branch on my pc anymore...

[fossdd]

Your Pull Request looks clean, simple. Thank you for your contribution.

[EdenOvrutskiy]

In the meanwhile, I'm working on the formats addition - and I've stumbled upon this quote -
at common.py , line 329:
"Unless mentioned otherwise, the fields should be Unicode strings."

I checked the format return statements in other extractors and they do in fact return u'' strings.

However, this pull request does not - should it? (given it returns the 'url' key and not formats as of yet)
Not relevant anymore since changed to formats (strikethrough instead of deleting for bugtracker documentation)

[EdenOvrutskiy]

Now has playlist functionality!