Line breaking behavior for function arguments with and without trailing comma

Hello and thanks for YAPF. It's been working nicely for us, except for the formatting of long function arguments. Take a look at the example below. If a trailing comma is present, we would like to keep the them in individual lines. Without trailing comma, an option to break right after the opening parenthesis would be nice. This is PEP8 conform (as is the the YAPF output) and plays more nicely with with long function names.

Desired format:

``` python
a_very_long_function_name(
long_argument_name_1=1,
long_argument_name_2=2,
long_argument_name_3=3,
long_argument_name_4=4,
)

a_very_long_function_name(
long_argument_name_1=1, long_argument_name_2=2,
long_argument_name_3=3, long_argument_name_4=4)
```

YAPF output:

``` python
a_very_long_function_name(long_argument_name_1=1,
long_argument_name_2=2,
long_argument_name_3=3,
long_argument_name_4=4, )

a_very_long_function_name(long_argument_name_1=1,
long_argument_name_2=2,
long_argument_name_3=3,
long_argument_name_4=4)
```