simple 'I', 'O', 'B' tagging throws error

Since the last commit, the simple 'I', 'O', 'B' tags are no longer supported.

```
from seqeval.metrics import f1_score

y_true = [['O', 'O', 'O', 'B', 'I', 'I', 'O'], ['B', 'I', 'O']]
y_pred = [['O', 'O', 'B', 'I', 'I', 'I', 'O'], ['B', 'I', 'O']]
f1_score(y_true, y_pred)
```

Throws error:
```ValueError: Invalid tag is found: B```

Is this behaviour intended?