Add XGlue
Dataset is ready to merge. An important feature of this dataset is that for each config the train data is in English, while dev and test data are in multiple languages. Therefore, @lhoestq and I decided offline that we will give the dataset the following API, *e.g.* for 

```python
load_dataset("xglue", "ner")  # would give the splits 'train', 'validation.en', 'test.en', 'validation.es', 'test.es', ... 
```
=> therefore one can load a single language test via

```python
load_dataset("xglue", "ner", split="test.es")
```

Close #749.