This also solves the issue of disappearing block content from top-level HTML blocks. Apparently gomarkdown does not provide a content-only attribute for HTML blocks; this is only for spans.

I think we could just strip out the tags from the block content using https://github.com/grokify/html-strip-tags-go; the issue of untrusted data is not important since clients should not be rendering HTML tags or JavaScript for gemtext! bluemonday could be used if additional sanitization is desired, but this is a heavier solution.

This corrects the issues with HTMLBlock and HTMLSpan parsing, adds a couple of features, and cleans up the code a bit.

For HTMLBlock only, <br> is now interpreted as a hard line break. This should present the content more closely to how it was originaly intended. This has not been implemented for HTMLSpan because it could cause issues with blockquotes, so this is reserved for another time.

For HTMLBlock only, the contents of several tags (script, iframe, etc) are stripped completely and will not be rendered. This can't be done as easily in HTMLSpan because the HTMLSpan only includes the tag itself, not the contents. It might be worth revisiting this later, although it's unlikely that many people will be including these tags inside of (for example) blockquotes or paragraphs.