Add `getType` override

[chrisvxd]

Currently you're forced to use the type for component type lookups. It would be good to have this overridable for use with systems like GraphQL, which natively expose the __typename property.

Something like:

const getType = obj => obj.__typename;

return <ReactFromJSON ... getType={getType} />

With the default value

const getType = obj => obj.type;