Add hashes to cnn_dailymail
The URL hashes are helpful for comparing results from other sources.
Looks you to me :)

Could you also update the json file that goes with the dataset script by doing 
```
nlp-cli test ./datasets/cnn_dailymail --save_infos --all_configs
```
It will update the features metadata and the size of the dataset with your changes. @lhoestq I ran that command.

Thanks for the helpful repository!