Add option to ignore words in BLEU

[aasseman]

No description provided.

[aasseman]

Not tested yet

[aasseman]

Test by printing the targets and predictions.
We go from that:

TARGET: ['viens', 'ici', 'immediatement', '.', '<EOS>', '<PAD>', '<PAD>', '<PAD>', '<PAD>', '<PAD>']
PREDICTION: ['.', '<PAD>', '<PAD>', '<PAD>', '<PAD>', '<PAD>', '<PAD>', '<PAD>', '<PAD>', '<PAD>']
BLEU: 0.4111336169005197

To:

TARGET: ['la', 'plupart', 'd', 'entre', 'nous', 'etait', 'des', 'veterans', '.']
PREDICTION: ['je', 'est', 'est', 'a', '.', '.']
BLEU: 7.060301868108111e-232

So I believe that it is working correctly.