Add smoothing function as a metric parameter for the BLEU text generation metric

[bnativi]

The smoothing function parameter was the most important parameter for Nanbo and I when we were researching data drift techniques. I think it will be common for users to not want smoothing and common for users to want smoothing, so I think this should be exposed to the user as an option.

[bnativi]

PR #742 adds the the bleu smoothing function as an evaluation parameter.