Add support for custom tokenizer for BLEU

[devrimcavusoglu]

Due to the nature of the Jury API, all input strings must be a whole (not tokenized), the current implementation of BLEU score is tokenized by white spaces. However, one might want results for smaller tokens, morphemes, or even character level rather than BLEU score of the words. Thus, it'd be great to support this with adding a support for tokenizer in the score computation for BLEU.