Sacrebleu

- Added Japanese MeCab tokenizer (`-tok ja-mecab`) (thanks to Makoto Morishita MorinoseiMorizo)
- Added wmt20/dev test sets (thanks to Martin Popel martinpopel)

- Smoothing changes (Sebastian Nickels sn1c)
- Fixed bug that only applied smoothing to n-grams for n > 2
- Added default smoothing values for methods "floor" (0) and "add-k" (1)
- `--list` now returns a list of all language pairs for a task when combined with `-t`
(e.g., `sacrebleu -t wmt19 --list`)
- added missing languages for IWSLT17
- Minor code improvements (Thomas Liao tholiao)

- Bugfix: handling of result object for CHRF
- Improved API example

- Tokenization variant omitted from the chrF signature; it is relevant only for BLEU (thanks to Martin Popel)
- Bugfix: call to sentence_bleu (thanks to Rachel Bawden)
- Documentation example for Python API (thanks to Vlad Lyalin)
- Calls to corpus_chrf and sentence_chrf now return a an object instead of a float (use result.score)

- Added sentence-level scoring via -sl (--sentence-level)

- Many thanks to Martin Popel for all the changes below!
- Added evaluation on concatenated test sets (e.g., `-t wmt17,wmt18`).
Works as long as they all have the same language pair.
- Added `sacrebleu --origlang` (both for evaluation on a subset and for `--echo`).
Note that while echoing prints just the subset, evaluation expects the complete
test set (and just skips the irrelevant parts).
- Added `sacrebleu --detail` for breakdown by domain-specific subsets of the test sets.
(Available for WMT19).
- Minor changes
- Improved display of `sacrebleu -h`
- Added `sacrebleu --list`
- Code refactoring
- Documentation and tests updates
- Fixed a race condition bug (`os.makedirs(outdir, exist_ok=True)` instead of `if os.path.exists`)