This moves Result and Error to mdbook-core with the anticipation of
using them in user crates. For now, the internal APIs will be using
anyhow directly, but the intent is to transition more of these to
mdbook-core where it makes sense.
* The preprocessor trait now returns a modified book instead of editing in place
* A preprocessor is told which render it's running for
* Made sure preprocessors get their renderer's name
* Users can now manually specify whether a preprocessor should run for a renderer
* You can normally use default preprocessors by default
* Got my logic around the wrong way
* Fixed the `build.use-default-preprocessors` flag
* Add index preprocessor
README.md is a de facto index file in markdown-based documentation.
Hence, we respect to README.md and convert it into index.html.
* Fix warning for unused variables
* Update tests for config
* Match file stem case-insensitively for IndexPreprocessor
* Add tests for IndexPreprocessor
* Update book example to fit index preprocessor
