allow to pass on an embedding matrix

[jwijffels]

Incorporate embeddings https://github.com/bnosac/crfsuite/blob/master/src/include/crfsuite_api.hpp#L147 at https://github.com/bnosac/crfsuite/blob/master/src/rcpp_crfsuite.cpp#L87

[jwijffels]

note that feature.minfreq can be a showstopper there as it will exclude features summing below zero