Add mdbook-course to handle parsing frontmatter (#1224)

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.

---------

Co-authored-by: Martin Geisler <martin@geisler.net>

mdbook-course/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "mdbook-course"
+version = "0.1.0"
+authors = ["Dustin Mitchell <djmitche@google.com>"]
+edition = "2021"
+license = "Apache-2.0"
+publish = false
+repository = "https://github.com/google/comprehensive-rust"
+description = "An mdbook preprocessor for comprehensive-rust."
+
+[dependencies]
+anyhow = "1.0.68"
+clap = "4.4.4"
+log = "0.4.17"
+matter = "0.1.0-alpha4"
+mdbook = "0.4.25"
+pretty_env_logger = "0.4.0"
+serde_json = "1.0.107"