Add ChrF++

[stancld]

What does this PR do?

This PR adds a new text evaluation metric - ChrF++.

Fixes #634

REF: https://github.com/m-popovic/chrF/blob/master/chrF%2B%2B.py

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

PR review

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If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

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[codecov]

Codecov Report

Merging #641 (728b8e3) into master (1fef5ae) will decrease coverage by 0%.
The diff coverage is 93%.

@@          Coverage Diff           @@
##           master   #641    +/-   ##
======================================
- Coverage      95%    95%    -0%     
======================================
  Files         159    161     +2     
  Lines        5584   5781   +197     
======================================
+ Hits         5315   5499   +184     
- Misses        269    282    +13     

[justusschock]

I cannot comment much on the implementation since I am not that much into NLP, but since you test that against another lib I assume it to be fine :)

Just a very minor question

[justusschock]

The missing question :D

[SkafteNicki]

only some typing issues remain :]
(please see the failing test for more info)