Support for outside-of-model feature engineering using model formulae

[vruusmann]

The r2pmml function has preProcess parameter, which encapsulates out-of-model feature engineering work that has been performed using the caret::preProcess() function.

However, most R users are more comfortable doing feature engineering using model formulae than the caret::preProcess() function. It would be nice if the r2pmml function also had a formula parameter, which would allow existing base::formula objects to be passed on to the converter.

[vruusmann]

User motivation: https://groups.google.com/d/topic/jpmml/Q85BNmhSHVQ/discussion

[sanchezismael]

I need this for save a PMML model with dummies transform.