As discussed in #3878 and #3886 , we might want a more extendable format for saving XGBoost model.

For now my plan is utilizing the JSONReader and JSONWriter implemented in dmlc-core to add experimental support for saving/loading model into Json file. Due to the fact that related area of code is quite messy and is dangerous to change, I want to share my plan and possibly an early PR as soon as possible so that someone could point out my mistakes earlier(there will be mistakes), and we don't make duplicated work. :)

@hcho3