adding masahaner dataset
Thank you for the review.  Thanks a lot for the corrections and comments. 

I have resolved point 2. The make style still throws some errors, please see below

black --line-length 119 --target-version py36 tests src benchmarks datasets/**/*.py metrics
/bin/sh: 1: black: not found
Makefile:13: recipe for target 'style' failed
make: *** [style] Error 127

Can you help to resolve this? Thank you very much @lhoestq for the help. 
Adding Masakhane dataset https://github.com/masakhane-io/masakhane-ner 

@lhoestq , can you please review