Add access to vocabulary in python bindings

[cypreess]

It would be nice to have access to kenlm.LanguageModel.vocab or even (maybe more pytonic way) to support iterable protocol on kenlm.LanguageModel.

[kpu]

Would a callback from LoadVirtual be sufficient?

[kpu]

The C++ side doesn't even remember the vocabulary strings by default because users either don't need it or have their own data structure populated by the EnumerateVocab callback API.

[cypreess]

I must say I did not read very deeply into the implementation. Just wondering if it's easy to implement access vocabulary somehow.

[manishbansal-fk]

@kpu Is there any way we can access LanguageModel vocab from python wrapper. I am loading model as kenlm.Model(model.klm) in python. "model.klm" is built from command line.