Ordinary reference as value ?

Now, gin config support `configurable reference`, `macro` , `basic type (str, bool, number)` or `nested of them` as value . It would be great if we can refer to local variable (module, class ,function)

```pthon
def testLocalVariableReference(self):
config_str = """
configurable2.non_kwarg = a
"""
config.parse_config(config_str)

a = 1
instance, _ = configurable2() # pylint: disable=no-value-for-parameter
self.assertEqual(instance, a)

a = 2
instance, _ = configurable2() # pylint: disable=no-value-for-parameter
self.assertEqual(instance, a)

def testUnregisteredFunction(self):
config_str = """
configurable2.non_kwarg = numpy.abs
"""
config.parse_config(config_str)
import numpy
instance, _ = configurable2() # pylint: disable=no-value-for-parameter
self.assertEqual(instance, numpy.abs)
```

without this , we make all function that may refer to to be configurable explicited by `gin.external_configurable` , it seems not reasonable .
see `gin.tf` and `gin.torch`