Support multiple zipped CSV data files

As requested by @lewtun, support loading multiple zipped CSV data files.

```python
from datasets import load_dataset

url = "https://domain.org/filename.zip"
data_files = {"train": "train_filename.csv", "test": "test_filename.csv"}
dataset = load_dataset("csv", data_dir=url, data_files=data_files)
```