[Multi Configs] Use directories to differentiate between subsets/configurations
Currently to define several subsets/configurations of your dataset, you need to use a dataset script.

However it would be nice to have a no-code way to to this. 

For example we could specify different configurations of a dataset (for example, if a dataset contains different languages) with one directory per configuration.

These structures are not supported right now, but would be nice to have:


```
my_dataset_repository/
├── README.md
├── en/
│   ├── train.csv
│   └── test.csv
└── fr/
    ├── train.csv
    └── test.csv
```

Or with one directory per split:

```
my_dataset_repository/
├── README.md
├── en/
│   ├── train/
│   │   ├── shard_0.csv
│   │   └── shard_1.csv
│   └── test/
│       ├── shard_0.csv
│       └── shard_1.csv
└── fr/
    ├── train/
    │   ├── shard_0.csv
    │   └── shard_1.csv
    └── test/
        ├── shard_0.csv
        └── shard_1.csv
```

cc @stevhliu @albertvillanova 

This can be specified in the README as YAML with

```
configs:
- config_name: en
  data_dir: en
- config_name: fr
  data_dir: fr
```