Add API to store private properties in any object

It's possible right now to create an "External Object" which stores a C pointer to internal native data. However, what's *not* possible as far as I can see is being able to store arbitrary properties on an object which refer to JS values but aren't accessible from JS. For example in Duktape it's possible to:

```c
duk_push_object(ctx);
/* populate new object with anything needed */
duk_put_prop_string(ctx, -2, "\xff""hiddenObject");
```

That can be done with any object, whereas in Chakra you currently have to create an external object to store private data--and it's not possible to convert an existing standard JS object to an external one, either, which means that in native constructors for external objects, you can't use the automatic `this` and must instead return a fresh object from the constructor.

I propose an API that would either:
* Add an API that converts an existing standard JS object to an External object, **OR**:
* Add an API for storing "hidden" properties that could be accessed using `Js[GS]etProperty()` but not within JavaScript code. Perhaps something like `JsCreateHiddenPropertyId()`.