Support the extensions field in errors

[tomhoule]

https://github.com/facebook/graphql/blob/master/spec/Section%207%20--%20Response.md

The question is whether we should use serde_json::Value, or a type parameter, or both (with two exports). I tend to favour the last solution. There is a complication with the GraphQLResponse type however, since it would need to take two type parameters.

[tomhoule]

#67 adds minimal support but it forces us to have serde_json as a dependency, which I don't know if I'm ok with.

[tomhoule]

In this PR we do not ensure the extensions field is a map, as said in the spec. We should change that.