add const access

[ahgamut]

It's not really necessary, but would be nice to have. The use case for me is the JSON object being part of a struct, and I have a bunch of const methods for the outer struct.

class Foo {
   jt::Json info;

   public:
        double getObj1A() { return info["obj1"].getNumber(); } // works
        double getObj1B() const { return info["obj1"].getNumber(); } // should work with this PR
        const std::vector<jt::Json>& getObj2() const { 
            return info["obj2"].getArray(); // should work with this PR
        }
}