Option to output digest manifest to JSON file.

Subsequent build steps could introspect this file, or a server could look up digested file paths to serve. On the path to #8.

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I say "on the path" because I didn't fully grok the desired changes for #8 once you had the manifest (why what's there "was a bad approach"). I took this simple step because the manifest I was looking for, for my own nefarious purposes, was already in memory as `renameMap`.

So let's discuss how to actually fulfill #8.