add kwargs to index search
This PR proposes to add kwargs to index search methods.

This is particularly useful for setting the timeout of a query on elasticsearch.

A typical use case would be:
```python
dset.add_elasticsearch_index("filename", es_client=es_client)
scores, examples = dset.get_nearest_examples("filename", "my_name-train_29", request_timeout=60)
```
_The documentation is not available anymore as the PR was closed or merged._