Relax HTML allowlist

[yhatt]

Marp Core's default HTML allowlist is only allowed <br> tag. It's definitely safe but but this default set is not matching with real use case. A lot of Marp users enjoy using custom HTML tags.

It is better to update default allowlist of HTML to include allowed elements in common Marp slides and Web services. e.g. <div>, <span>, and HTML elements that are converted by Markdown.

https://github.com/gjtorikian/html-pipeline/blob/HEAD/lib/html/pipeline/sanitization_filter.rb is mentioned as GitHub style sanitization from some famous Markdown parsers. (remark, marked-sanitizer-github)

[nikhilweee]

I would advocate for allowing <span> tags as then I can use different colors for different parts of the slide.