Add XGlue
Really cool to add XGlue, this will be a nice addition !

Splits shouldn't depend on the language. There must be configurations for each language, as we're doing for xnli, xtreme, etc.
For example for XGlue we'll have these configurations: NER.de, NER.en etc.
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.