Add Matthew's Correlation Coefficient as an eval metric

[lukaszdz]

I would like to implement this and expose it in the Python API: https://bmcgenomics.biomedcentral.com/articles/10.1186/s12864-019-6413-7

cc @EdouardGrave, guessing I could pretty much follow your work here?: https://github.com/facebookresearch/fastText/commit/2cc7f54ac034ae320a9af784b8145c50cc68965c