Add CHRF

[devrimcavusoglu]

Brief metric info
chrF++ is a tool for automatic evaluation of machine translation output based on character n-gram precision and recall enhanced with word n-grams. The tool calculates the F-score averaged on all character and word n-grams, where the default character n-gram order is 6 and word n-gram order is 2. The arithmetic mean is used for n-gram averaging.

Meta information
Please fill below if applicable:

• Repo/Custom implementation: https://github.com/m-popovic/chrF
• Python implementation available: [x]
• Paper: https://aclanthology.org/W15-3049
• Huggingface: https://github.com/huggingface/datasets/tree/master/metrics/chrf