Add support for CTranslate2 backend

[lewoudar]

Hi everyone,
first of all, I want to thank the author for this nice library.
It would be interesting if the project supports Ctranslate2. It promises some huge performance improvements.

[rlouf]

We could, it looks like the model forward pass is exposed with the Python bindings: https://opennmt.net/CTranslate2/python/ctranslate2.Generator.html#ctranslate2.Generator.generate_tokens. Although that would represent more work than with e.g. the llama.cpp integration.