Implement common data loader to support csv/libsvm in all inferencing

[jfomhover]

Lightgbm_python and treelite_python both consume inference data.

That data could be either csv or libsvm, but because we're using numpy in treelite_python, it doesn't support libsvm.

To avoid writing the same data loading code twice, we want to implement a common class for handling that through either numpy or libsvm or lightgbm itself.

[jfomhover]

This is less relevant now that the LightGBM C API uses the LightGBM Parser #153