For objects, add support for `operator[]( const char*)`

[FFMG]

Is your feature request related to a problem? Please describe.

We need another easier way to access the data for object.

Currently we can do json.get<int>( key ); as well as json.get_*( key ); but some users might like json[ key ].as<int>();

Might also be nice to be able to access by number, (even for objects).

Describe the solution you'd like

Maybe look into adding it to TJValue directly and we could smartly have a virtual operator[...] that would be able to get the value and it could then be cast as<T> ...