Adds an AddFieldConfig method to InputObjects

[robdaemon]

Thunks don't always work for adding self-referential fields
to an InputObject.

Example:

inputObj := graphql.NewInputObject(graphql.InputObjectConfig{
    Name: "MyObject",
    Fields: (graphql.InputObjectConfigFieldMapThunk)(func() graphql.InputObjectConfigFieldMap {
        return graphql.InputObjectConfigFieldMap{
            "parent": &graphql.InputObjectFieldConfig{
                Type: inputObj,
            },
        }
    }),
})

Go does not this definition, but you can work around this with AddFieldConfig.

This makes an InputObject work like other Object types.

[coveralls]

Coverage decreased (-0.06%) to 91.74% when pulling a05d5bb on robdaemon:add_inputobject_field into 83f0349 on graphql-go:master.

[chris-ramon]

LGTM 👍 — thanks a lot @robdaemon