Adding youtube embed video plugin

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/docs/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [ ] Update to the latest Gem version (run `bundle update github-pages`)

### This issue affects

- [x] The site generated by GitHub Pages
- [ ] Building sites locally

*The GitHub Pages Gem is intended to help users replicate the GitHub Pages build environment locally. If your issue affects both the hosted version and the version previewed locally, you may be better suited reporting [seeking support in other forums](https://github.com/github/pages-gem/blob/master/docs/SUPPORT.md).*

### What did you do (e.g., steps to reproduce)

Add https://github.com/dommmel/jekyll-youtube to the plugin
### What did you expect to happen?

### What happened instead?

It worked locally but failed on github pages due to the fact that the plugin is not considered as safe.

### Additional information

* Link to the live site (if applicable):
* Link to the source repo (if applicable): https://github.com/dommmel/jekyll-youtube

The development is not very active but the project is so simple that it just works. I'm currently creating a pull request to improve it.

Überprüfe die Anweisungen in CONTRIBUTING.md des Repositorys und den in der Issue verlinkten jekyll-youtube-Quellcode. Ermittle zunächst, wie GitHub Pages mit sicheren Jekyll-Plugins umgeht, und verifiziere anschließend, dass das Plugin für gehostete Builds akzeptiert werden kann. Die Aufgabe ist abgeschlossen, wenn das YouTube-Embed-Plugin eingebunden ist, ohne die Sicherheitsprüfungen von GitHub Pages zu verletzen.

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