Add Wikimedia pre-processed datasets
## Adding a Dataset
- **Name:** Add pre-processed data to:
  - *wikimedia/wikipedia*: https://huggingface.co/datasets/wikimedia/wikipedia
  - *wikimedia/wikisource*: https://huggingface.co/datasets/wikimedia/wikisource
- **Description:** Add pre-processed data to the Hub for all languages
- **Paper:** *link to the dataset paper if available*
- **Data:** *link to the Github repository or current dataset location*
- **Motivation:** This will be very useful for the NLP community, as the pre-processing has a high cost for lot of researchers (both in computation and in knowledge)

Instructions to add a new dataset can be found [here](https://github.com/huggingface/datasets/blob/master/ADD_NEW_DATASET.md).

CC: @geohci, @yjernite