LightGBM

[Python] / [R] add start_iteration to python predict interface (fix #3058) (#3272)

* [python] add start_iteration to python predict interface (#3058)

* Apply suggestions from code review

* Update lightgbm_R.h

* Apply suggestions from code review

* Apply suggestions from code review

* fix R interface

* update R documentation

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