This adds the `optional` field to the preprocessor configuration to
mirror the same option for the `output` table. Missing preprocessors are
now an error unless the `optional` field is set. This should help with
inadvertently building a book when a missing preprocessor that you
expect to be installed.
This changes preprocessors so that:
- Relative paths in the `command` value are relative to the book root.
- The process current directory is the book root.
This makes it so that it isn't dependent on the directory where `mdbook`
is executed.
Fixes https://github.com/rust-lang/mdBook/issues/1424
This switches all public types to use non_exhaustive to make it easier
to make additions without a semver-breaking change.
Some of the ergonomics are hampered due to the lack of exhaustiveness
checking. Hopefully some day in the future,
non_exhaustive_omitted_patterns_lint or something like it will get
stabilized.
Closes https://github.com/rust-lang/mdBook/issues/1835
