Add support for categorical feature auto-encoding

[nunosilva800]

Adds support to have categorical features label encoded automatically, when using Booster#predict.

The python api does it by relying on pandas dataframes: https://github.com/microsoft/LightGBM/blob/e057ae08e6bf6c6c84f276a127423fb145ca5fdb/python-package/lightgbm/basic.py#L1131-L1137

Would be great for this ruby lib to provide similar quality-of-life.