new text analytics functions

[shovsj]

several text analytics functions will be developed in this issue.(including #102)

• analyzer using mecab
• tokenizers using mecab, twkorean(including lemmatization, normalization)
• wordcloud
• doc2vec, doc2vec model
• possibly tfidf, naivebayes, word2vec, fasttext, ...

[shovsj]

please make a PR to branch brtc-issue-107.

[krazyeom]

Added several functions.

• ngram
• lda
• tfidf
• tfidf2
• tokenizer Eng
• tokenizer Kor
• bow
• doc-doc matrix
• doc-term matrix
• term-term matrix
• stopwords remover
• synonym converter
• word2vec
• doc2vec
• search