Better support for representing default values in introspection

[mwildehahn]

#102 (comment)

I started doing this but input objects are a little tricky so going to do this as a PR on it's own.

[jjergus]

input objects are a little tricky

Yeah there might be a bit of a cyclical dependency there, you need the types from the InputValue shapes to output the default values, and you need the default values for the InputValue shapes :)