Lambada
Added LAMBADA dataset.

A couple of points of attention (mostly because I am not sure)
- The training data are compressed in a .tar file inside the main tar.gz file. I had to manually un-tar the training file to access the examples.
- The dev and test splits don't have the `category` field so I put `None` by default.

Happy to make changes if it doesn't respect the guidelines!

Victor