This adds a test to ensure that the interface for preprocessors and
renderers does not change unexpectedly, particularly in a semver
compatible release.
Closes https://github.com/rust-lang/mdBook/issues/1574
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.
