Add CBT dataset
Hi @lhoestq,

I have added changes from the review. Thanks for approving @lhoestq 
This PR adds the [CBT Dataset](https://arxiv.org/abs/1511.02301).

Note that I have also added the `raw` dataset as a separate configuration. I couldn't find a suitable "task" for it in YAML tags.

The dummy files have one example each, as the examples are slightly big. For `raw` dataset, I just used top few lines, because they are entire books and would take up a lot of space.

Let me know in case of any issues.