LightGBM

added feature infos to JSON dump (#2660)

* added feature infos to JSON dump

* slight json schema refactor

* simpified code

* refactor feature_infos

* refactoring

* Update src/boosting/gbdt.cpp

* Update dataset.h

* Update include/LightGBM/dataset.h

* simplify

* Apply suggestions from code review

* parse string and construct JSON objs

Co-authored-by: Guolin Ke <guolin.ke@outlook.com>