Publicly re-export mdbook-core modules from mdbook-driver

The intent here is to make mdbook-core a private dependency that the
user shouldn't need.

crates/mdbook-driver/src/lib.rs

@@ -7,3 +7,4 @@ mod load;
 mod mdbook;
 
 pub use mdbook::MDBook;
+pub use mdbook_core::{book, config, errors};