Support mdBook preprocessors for TRPL in rustbook

`rust-lang/book` recently added two mdBook preprocessors. Enable
`rustbook` to use those preprocessors for books where they are requested
by the `book.toml` by adding the preprocessors as path dependencies, and
ignoring them where they are not requested, i.e. by all the books other
than TRPL at present.

Author: chriskrycho

src/doc/book

@@ -8,6 +8,9 @@ use clap::{arg, ArgMatches, Command};
 use mdbook::errors::Result as Result3;
 use mdbook::MDBook;
 
+use mdbook_trpl_listing::TrplListing;
+use mdbook_trpl_note::TrplNote;
+
 fn main() {
     let crate_version = concat!("v", crate_version!());
     env_logger::Builder::from_env(env_logger::Env::default().default_filter_or("warn")).init();
@@ -67,6 +70,14 @@ pub fn build(args: &ArgMatches) -> Result3<()> {
         book.config.build.build_dir = dest_dir.into();
     }
 
+    if book.config.get_preprocessor("trpl-note").is_some() {
+        book.with_preprocessor(TrplNote);
+    }
+
+    if book.config.get_preprocessor("trpl-listing").is_some() {
+        book.with_preprocessor(TrplListing);
+    }
+
     book.build()?;
 
     Ok(())