Add map and flatmap methods

[TomVanWemmel]

For evaluating nested structures, it would be a nice addition to the JsonNullable class to introduce JsonNullable<U> map(Function<T,U> mapper) and JsonNullable<U> flatMap(Function<T, JsonNullable<U>> mapper) methods like Optional has.

There has been a pull request for this functionality:
#56

Is there any particular reason why this hasn't been considered?