prefer splitting function args to splitting on indexing

I ran across this example that I think behaves poorly. Take some nicely formatted case:

``` python
dictionary_with_really_really_long_name_or_something[index].update(
other_dict['other_value']
)
```

it will format to

``` python
dictionary_with_really_really_long_name_or_something[index].update(other_dict[
'other_value'])
```

I think splitting on function args "(" is easier to read than splitting on indexing "[".