* 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
* First version of preprocessor example, with quicli
It seems it's not worth it right now.
* Remove quicli, just to simplify everything
* Finish de-emphasise example
* Finish preprocessor example in book
* Rename preprocessor type
* Apply changes requested in review
* Update preprocessor docs with latest code
[skip CI]
