Additional allow attributes & tags

[dab246]

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[jonasfj]

At the moment this package is aimed at aligning with github gfm sanitization rules:
https://github.com/jch/html-pipeline/blob/master/lib/html/pipeline/sanitization_filter.rb

Arguably, these have changed, I don't think github uses that code anymore.

But I'm hesitant to just add features. If there is a lot of community members that would rather have an HTML sanitation package with more advanced features I'm inclined to suggest that you write such a package, publish and maintain it yourself.

Feel free to fork this package and give it a new better name, ideally collaborate with others.
I see that @jawad111 has suggestions in #263.

Honestly, I'd be more than happy for package:sanitize_html to have a link in the README.md telling users that if they want advanced features they should consider one of the following packages... (assuming you make a good package obviously 🤣)

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On topic of this PR, I might get around to doing a review, but again, I'm hesitant to accept more features. Maybe, if it turns out we need them elsewhere too.