allow yaml as config file extension

[niekdomi]

yaml is actually the preferred file extension, see:

https://web.archive.org/web/20210311073506/https://yaml.org/faq.html

or

https://stackoverflow.com/a/22268973

Though I kept yml as default to prevent potential breaking changes

I haven't updated the docs yet (Its just Config.md i think), since I want to gather some feedback first.

PR Description

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary TODO
• You've read through your own file changes for silly mistakes etc

[codacy-production]

Up to standards ✅

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