support for episodes in english on southpark.de

[TH42]

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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

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Description of your pull request and other information

The southpark extractor can't handle episodes with english language track on southpark.de. Links to episodes with english language track contain 'en/episodes' instead of 'folgen' in their urls. The same might be true for clips and collections. Added 'en/episodes' to be recognized as a valid url.