Support OpenNMT models
It will be great if OpenNMT (https://opennmt.net/) and CTranslate2 (https://github.com/OpenNMT/CTranslate2) model support is provided out of the box. 
Hi @jordimas !

Would you be interested in adding this model to transformers? I briefly looked at the code and it looks similar to [mbart](https://github.com/huggingface/transformers/blob/master/src/transformers/models/mbart/modeling_mbart.py)/[m2m](https://github.com/huggingface/transformers/blob/master/src/transformers/models/m2m_100/modeling_m2m_100.py)/[Marian](https://github.com/huggingface/transformers/blob/master/src/transformers/models/marian/modeling_marian.py) style models in transformers. So it should be fairly straightforward to port this by looking at the design of these models.

As you might already know, each model in the library requires 3 files, which will look something like this:
- `configuration_open_nmt.py`
- `modeling_open_nmt.py`
- `tokenization_open_nmt.py`


We provide a template using CookieCutter which lets you set up these files for you, even filling in the names, as explained [here](https://github.com/huggingface/transformers/tree/master/templates/adding_a_new_model). It also creates documentation pages, test files, and so on.

I would be happy to help with this, so feel free to ping me if you have any issues. Thank you :) 