BLEU: add COCO/PTB tokenization mode (tokenizer_name) for parity with pycocoevalcap

[bavi404]

fixes #693

Summary

• Add tokenizer_name option to bleu metric.
• New modes: "coco"/"ptb" (COCO PTBTokenizer; requires pycocoevalcap) and "whitespace".
• Default behavior unchanged (tokenizer_13a).

Motivation

• Address discrepancy with pycocoevalcap BLEU caused by PTB vs 13a tokenization differences, esp. around commas/periods.

Changes

• metrics/bleu/tokenizer_13a.py: add CocoPTBTokenizer, WhitespaceTokenizer.
• metrics/bleu/bleu.py: new tokenizer_name param; accepts "coco"|"ptb"|"whitespace".
• metrics/bleu/README.md: document new option and example.
• tests/test_bleu_coco_tokenization.py: focused tests; skipped if pycocoevalcap not installed.

Backwards compatibility

• Default tokenizer unchanged; no breaking API changes.

Notes

• COCO/PTB mode matches the issue’s reported scores.