Implement a markdown renderer (#1018)

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.

README.md

@@ -169,6 +169,9 @@ format, however there's nothing stopping a renderer from doing static analysis
 of a book in order to validate links or run tests. Some existing renderers are:
 
 - `html` - the built-in renderer which will generate a HTML version of the book
+- `markdown` - the built-in renderer (disabled by default) which will run
+  preprocessors then output the resulting Markdown. Useful for debugging
+  preprocessors.
 - [`linkcheck`] - a backend which will check that all links are valid
 - [`epub`] - an experimental EPUB generator