Lightgbm: can add a mape objective function in lightgbm?

[hlee13]

Such as the title.
reference to：https://en.wikipedia.org/wiki/Mean_absolute_percentage_error

could show the mape objective function code segment in python now.

[wxchan]

check https://github.com/dmlc/xgboost/issues/1818

[Laurae2]

@hlee13 You can contribute by adding MAPE to LightGBM in C++, or you can use a custom metric in R/Python.

[guolinke]

added in #1199