Add features parameter to CSV

[lhoestq]

Add support for the features parameter when loading a csv dataset:

from datasets import load_dataset, Features

features = Features({...})
csv_dataset = load_dataset("csv", data_files=["path/to/my/file.csv"], features=features)

I added tests to make sure that it is also compatible with the caching system

Fix #623

[thomwolf]

Very clean!

[thomwolf]

Good catch!