[FEATURE REQUEST] user param support.

[noizu]

For some of my scaffolding libraries I conditionally jason encode/decode entities to shape output for caller's preferences, and to strip/control access to restricted fields.

To do this I need to be able to pass custom environment flags in the the encoder call, or (assuming no encoders spawn off of the initiating process) track details in the process env.

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I have a rough patch here although I think there may be better/more efficient solutions here.

#178