Add support for watching additional directories

[willcrichton]

When using mdBook preprocessors like mdbook-quiz, the book depends on files outside the source directory, such as TOML files describing quizzes. It would be very convenient as an author for the book to automatically rebuild when these files change. However, the watch and serve commands (via trigger_on_change) only watch a fixed set of directories.

Probably the ideal solution would be for preprocessors to register files to be watched, similar to how extensible build systems like esbuild manage plugins. However, this would be a big change with a lot of work. A much cheaper solution is to allow mdbook CLI users to pass desired directories-to-watch on the command line.

Therefore this PR adds a new flag to watch and serve, --extra-watch-dirs. For example, a user could write:

mdbook serve --extra-watch-dirs quizzes,snippets

The --extra-watch-dirs flag takes as input a comma-separated list of directories. Each of these directories is added to the file watcher, so the book is regenerated when a file in these directories is changed.

[ehuss]

Might it be better to add this to book.toml instead of a CLI flag? That way, all users will get the benefit of proper watch behavior without needing to know a specific CLI flag to pass?

[willcrichton]

Ah yeah that's a good suggestion. Plus it makes the implementation simpler! Now the configuration looks like:

[build]
extra-watch-dirs = ["quizzes", "snippets"]

[ehuss]

Thanks! It looks like CI is failing?

Also, when adding new config options, the documentation needs to be updated, too.

[willcrichton]

Ah, didn't realize that CI was using an older version of Rust. That's been fixed. I just added documentation.

[ehuss]

Thanks!