Add nlp class api

[wannaphong]

Detailed description

Add nlp class api to use PyThaiNLP. The nlp class will look like spaCy's nlp. Now, PyThaiNLP have many high-level function because PyThaiNLP have many fetaures, so I think we should have simple api for beginner and intermediate.

Context

nlp class is the simple api for using PyThaiNLP. It will don't have the advanced parameter and many api, so still keep main function only in the class look like pos_tag, ner, and more in one class.