Register special function to formula interface

[wush978]

Currently, the FeatureHashing supports split in the formula to split the string before generating the model matrix. Here, the split is called special in the formula according to the documentation of terms.formula.

A more general approach is to let the user register their own special in the formula interface and invoke them before generating the model matrix.

Then, the request of n-gram (#66 (comment)) could be done via register the n-gram generator to FeatureHashing.

[pommedeterresautee]

Is there some pointer somewhere to help implement that feature?