Evaluate reference with kwargs ?

Sometimes it is necessary to pass the result of calling a specific function or class constructor. Gin supports "evaluating" configurable references via the @name() syntax, but do not support passing
kwargs , it's not flexible

For example, say we wanted to use the result of calling `test_func`

```python
@gin.configurable
def test_func(a , b)
...
```
we should config as
```
scope/test_func.a='a'
scope/test_func.b='b'
ref=scope/test_func()
```

should it possible be simplified as
```
ref=scope/test_func(a='a', b='b')
```