Add support for utoken tokenizer

[ddaspit]

utoken is a general-purpose word tokenizer, in the spirit of sacremoses. Machine can provide a utoken implementation of the tokenizer interface.

[johnml1135]

@ddaspit - is this still something we want to do? If not, let's close the issue.

[ddaspit]

Yes, we still want to do this at some point. It would be done to make it available in the library.