class_update: check if there are idf values associated with model, before applying weights
estimator: make use of preproc() function for data preprocessing
preproc: function containing all logic with regards to text data preprocessing and weighting
dfm_gen: added old junk codes for recoding, and removed deprecated ngrams parameter from dfm function
class_update: removed dfm_words parameter, which is replaced by the force = T parameter in predict(), training/model idf is now applied to unseen data
DESCRIPTION: added quanteda.textmodels as new dependency, since these have been separated from base quanteda 2.0.0 onwards