Adding Igbo monolingual dataset
@lhoestq Thank you for the review. I have made all the changes you mentioned. PTAL! 
This PR adds the Igbo Monolingual dataset.
Data: https://github.com/IgnatiusEzeani/IGBONLP/tree/master/ig_monoling
Paper: https://arxiv.org/abs/2004.00648  