This makes the renderer print the content of informational
HTML tags while stripping the tags themselves.
Tags like script, iframe, style, etc, which are unlikely to
ever hold presentable content, are exempt from this, and
their content is skipped from rendering as well as the tags
themselves.
<br>, a hard-break tag, is supported as a Markdown
hard-break replacement (the two spaces before newline).
This also adds tests for this behavior inside general_text.md.
Fixes#6, a longstanding issue with inline HTML in
blockquotes.
ddb3943359