Manual genre should support separate metadata for separate passes

[david-christiansen]

Right now, metadata on custom blocks or inlines is saved as JSON. But often only a subset of it is needed in the traversal or HTML generation passes.

This is easy enough to work around in an ad-hoc manner by saving e.g. a JSON object with serialized info, then projecting the fields that matter. But it'd be better if the API made all of this easier - perhaps the incoming info should be a NameMap Json instead?