[Twitch] collection support

[cromefire]

• I've verified and I assure that I'm running youtube-dl 2019.03.18

Before submitting an issue make sure you have:

• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones
• Checked that provided video/audio/playlist URLs (if any) are alive and playable in a browser

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

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If the purpose of this issue is a site support request please provide all kinds of example URLs support for which should be included (replace following example URLs by yours):

• Playlist: https://www.twitch.tv/collections/KFti877JixXiHQ (the first I found)

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Description of your issue, suggested solution and other information

See the url above for a random collection example, a collection is Twitch's version of a "playlist", currently ytdl thinks it's a streamer

ERROR: collections is offline

[mtilbury]

Hi, I've confirmed this issue still exists with the latest version, and specifying the --yes-playlist flag still results in the same error. It looks like there just isn't an information extractor to handle collections. I'm going to work on this.

[Kcee8]

Is this still being worked on?

[cromefire]

The PR is ready someone would just need to merge it

[zouhair]

This would be a nice thing to have

Now I keep getting "ERROR: collections is offline" for all collections