[Feature request] Add Toronto BookCorpus dataset
As far as I understand, `wikitext` is refer to `WikiText-103` and `WikiText-2` that created by researchers in Salesforce, and mostly used in traditional language modeling.

You might want to say `wikipedia`, a dump from wikimedia foundation.

Also I would like to have Toronto BookCorpus too ! Though it involves copyright problem... Hi, @lhoestq, just a reminder that this is solved by #248 .😉 
I know the copyright/distribution of this one is complex, but it would be great to have! That, combined with the existing `wikitext`, would provide a complete dataset for pretraining models like BERT.