Remove nltk dependency?

[eginhard]

NLTK is required by g2pkk only to load the CMU lexicon data. It might be better to include the lexicon file in this repository and load it manually in g2pkk. Then g2pkk and Coqui TTS don't need to depend on the entire NLTK library.

[harmlessman]

I saw your comment and thought about using nltk. It seems possible to remove the dependency of nltk by including nltk's cmudict file in g2pkk.

Thank you very much if you post it on pull requests.
or not, If I have time, I'll fix it.