Use case: when trying to `mdbook test` a file that has many `include`
directives, and a test fails, the line numbers in the `rustdoc` output
don't match the line numbers in the original markdown file.
Turning on the markdown renderer implemented here lets you see what is
being passed to `rustdoc` by saving the markdown after the preprocessors
have run.
This renderer could be helpful for debugging many preprocessors, but
it's probably not useful in the general case, so it's turned off by
default.
Carol (Nichols || Goulding)