Add `getType` override

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:

```jsx
const getType = obj => obj.__typename;

return <ReactFromJSON ... getType={getType} />
```

With the default value

```jsx
const getType = obj => obj.type;
```