* 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
* Created regression tests for the table of contents
* Refactoring to make the test more readable
* Fixed some bitrot and removed the (now redundant) tests/helper module
* Removed the include_str!() stuff and use just the dummy book for testing
* Regression tests now pass again!
* Pinned a `*` dependency to use a particular version
* Made sure test mocks return errors instead of panicking
* Addressed the rest of @budziq's review
* Replaced a file open/read with file_to_string
Move non-test test module files into their own directories to prevent
cargo from running them as tests. Then suppress the left-over warnings.
Move *dummy book* code and data into a shared folder, and leave the rest
of helper utilities (one function) in the original module.
