How much do we hide the JSON nature of the underlying representation?

[DougBurke]

This is related to #4 but worth a separate issue.

At present the types are a "thin shim" around the Aeson/JSON representation. This allows some flexibility which could be useful - such as manually adding or changing elements if the VegaLite schema has changed from that supported by this module - but ends up feeling like you are just using Map Text Text with little guidance from the types. i do not have a suggestion for what a more-typed alternative would be.