How about addding custom word_tokenizers?

How about addding custom word tokenizer class in `utis/word_tokenizers.py`?

the reason is following:
+ I just want not to use determined tokenizer(in `word_tokenizers.WORD_TOKENIZER_FACTORY`) but other tokenizer(such as [khaiii](https://github.com/kakao/khaiii)).
+ Some other languages can make their own tokenizer with custom tokenizer class.

Start with `utis/word_tokenizers.py` and inspect `word_tokenizers.WORD_TOKENIZER_FACTORY` to understand how tokenizers are selected. Consider how a custom class such as khaiii's tokenizer could be supplied and how other languages could provide their own implementations. Done should mean callers can use custom tokenizers instead of only the existing determined choices.

Written by the indexing model from the issue text.