Implement Index for Object

[hobofan]

Sometimes I like to pass around JSON Object's after filtering them out of a bigger JSON structure.

As it stands right now, if I want to access its fields with the same easy indexing syntax, I first have to put i JsonValue again, which feels kind of pointless.

Would you accept a PR that also implements Index directly for Object?

[maciejhirsz]

That sounds perfectly reasonable and should be pretty easy to do. If you'd like to commit a PR for that I'd more than welcome it :).