relative preprocessor paths are relative to the current directory instead of the book root

[ehuss]

If a preprocessor has a custom command path like foo/bar, then the path will be relative to the current directory. I think it should be relative to the book root. This only really matters if mdbook is run outside of the book root.

Unfortunately this is not easy to change without a breaking API change. CmdPreprocessor could add a root field, or Preprocessor::supports_renderer trait method would need to pass in the book root.

See #1418 for more related to backend relative paths.