GitLab

* NerPipeline (TokenClassification) now outputs offsets of words

- It happens that the offsets are missing, it forces the user to pattern
match the "word" from his input, which is not always feasible.
For instance if a sentence contains the same word twice, then there
is no way to know which is which.
- This PR proposes to fix that by outputting 2 new keys for this
pipelines outputs, "start" and "end", which correspond to the string
offsets of the word. That means that we should always have the
invariant:

```python
input[entity["start"]: entity["end"]] == entity["entity_group"]
                                    # or entity["entity"] if not grouped
```

* Fixing doc style