[NLP] Add Tf-Idf and count vectorizers

[mratsim]

See https://scikit-learn.org/stable/modules/generated/sklearn.feature_extraction.text.TfidfVectorizer.html

A very nice introduction to TF-IDF is available here: https://cran.r-project.org/web/packages/tidytext/vignettes/tf_idf.html

Efficient vectorization probably requires sparse matrices support (#28)