Must Run as First Preprocessor when Summary Preprocessing is Enabled

[ssanderson]

Currently, if summary preprocessing is enabled, we essentially re-parse the entire mdbook when we're invoked. This discards the results of any preprocessors that run before us, which means we effectively need to be the first preprocessor that runs.

Ideally mdbook would provide an interface by which we could evaluate all of SUMMARY.md as a template before it gets used by the rest of the mdbook machinery. This requires changes within mdbook itself: rust-lang/mdBook#2466