* 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
