Adding nli_tr dataset
Thank you @lhoestq for the time you take to review our pull request. We appreciate your help.

We've made the changes you described. Hope that it is ready for being merged. Please let me know if you have any additional requests for revisions. 
Hello,

In this pull request, we have implemented the necessary interface to add our recent dataset [NLI-TR](https://github.com/boun-tabi/NLI-TR).  The datasets will be presented on a full paper at EMNLP 2020 this month. [[arXiv link] ](https://arxiv.org/pdf/2004.14963.pdf)

The dataset is the neural machine translation of SNLI and MultiNLI datasets into Turkish.  So, we followed a similar format with the original datasets hosted in the HuggingFace datasets hub.  

Our dataset is designed to be accessed as follows by following the interface of the GLUE dataset that provides multiple datasets in a single interface over the HuggingFace datasets hub.  

```
from datasets import load_dataset
multinli_tr = load_dataset("nli_tr", "multinli_tr")
snli_tr = load_dataset("nli_tr", "snli_tr")
```

Thanks for your help in reviewing our pull request.