In v2 of the course, I'd like to include an estimate of the time to be
spent on each segment in the Markdown file. I think a good place for
such metadata is in the frontmatter.
For review purposes, though, I just want to display that information.
So, this is a start at a new mdbook preprocessor that just separates out
the frontmatter and includes it in a `<pre>` block. Eventually, I'd like
to parse it and put the time in the speaker notes.
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Co-authored-by: Martin Geisler <martin@geisler.net>