Add Xgboost into models

[nnarukulla]

Thank you for developing this package. This is awesome. Can we add xgboost into this hybridforecast. Most of the winning solutions for forecasting competitions having xgboost models in them. So that this package covers all time series models plus advanced machine learning algorithms (nnetar and xgboost).

[dashaub]

@nnarukulla For this to work, we'd have to build useful features from the timeseries so that xgboost can train on it. Do you have some example code for the types of features that xgboost trains well on? Really this might be well suited to a separate package first and then we can import it and integrate it from there.

[dashaub]

Also this would break our prediction intervals (which we finally gained for all the models in our ensemble).

[ellisp]

I've started work on a separate package at https://github.com/ellisp/forecastxgb-r-package. Very early days yet but looks promising.