Allow picking keys during export

[umesh-timalsina]

The WJI format can get quite verbose and the toJSON sometimes contains properties which are not necessarily relavent to the client application. Hence, can the signature toJSON(node, shallow) be modified to toJSON(node, attributes=['children', 'pointers', ....])? I am fine with the previous signature or a signature like toJSON(node, omitAttributes=['children', 'pointers', ...])

[brollb]

This sounds good but it might be nice to use a Set rather than an index as I would expect it to have better performance than using an array (and the position doesn't have any meaning here).