Add large spanish corpus
Adds a collection of Spanish corpora that can be useful for pretraining language models. 

Following a nice suggestion from @yjernite we provide the user with three main ways to preprocess / load either 

* the whole corpus (17GB!)
* one specific sub-corpus
* the whole corpus, but return a single split. this is useful if you want to cache the whole preprocessing step once and interact with individual sub-corpora

See the dataset card for more details.

Ready for review!