Mutual Information

參考連結： https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-significantterms-aggregation.html#search-aggregations-bucket-significantterms-aggregation http://ccckmit.wikidot.com/st:mutualinformation http://cpmarkchang.logdown.com/posts/195584-natural-language-processing-pointwise-mutual-information