String Processing Utilities - Week 10

So last few weeks haven't been that productive, and hence I am lagging behind a lot, This week I added BOW encoding algorithm and also Tf-IDF implementation to lozhnikov's branch and made a fresh PR. After researching a bit we came to know that there are different varieties of TF-IDF algorithm and hence we implemented all of those.

Also I made minor correction in 1904PR, it is almost finished, I just need to handle a few more comments, and also have to break it down into small function, probably I will finish both 1904 and TF-IDF and Bow and then can dedicate my last 2 weeks for Word2Vec algorithm, I am sure about its API to fit the existing API, but I am sure we will come up with some workaround.

Thank you :)