don't require min_chunk_size on Creator

Currently, `libzim.writer.Creator` requires all its arguments to be passed:

```py
def __init__(self, filename, main_page, index_language, min_chunk_size):
```

While it's understandable for `filename` and `main_page`, and debatable for `index_language`, `min_chunk_size` is definitely an advanced feature and should receive a default value.