Add metrics
This PR is a draft for adding metrics (sacrebleu and seqeval are added)

use case examples:
`import nlp`
**sacrebleu:**
```
refs = [['The dog bit the man.', 'It was not unexpected.', 'The man bit him first.'],
        ['The dog had bit the man.', 'No one was surprised.', 'The man had bitten the dog.']]
sys = ['The dog bit the man.', "It wasn't surprising.", 'The man had just bitten him.']
sacrebleu = nlp.load_metrics('sacrebleu')
print(sacrebleu.score)
```

**seqeval:**
```
y_true = [['O', 'O', 'O', 'B-MISC', 'I-MISC', 'I-MISC', 'O'], ['B-PER', 'I-PER', 'O']]
y_pred = [['O', 'O', 'B-MISC', 'I-MISC', 'I-MISC', 'I-MISC', 'O'], ['B-PER', 'I-PER', 'O']]
seqeval = nlp.load_metrics('seqeval')
print(seqeval.accuracy_score(y_true, y_pred)
print(seqeval.f1_score(y_true, y_pred)
```
_examples are taken from the corresponding web page_

your comments and suggestions are more than welcomed



