Support resolution of configs defined in function defaults

[JIy3AHKO]

I think it would reduce code duplication, if we could specify all config values in function signature:

import cfg_package

@cfn.config()
def func(arg=1, configurable_arg=cfg_package.some_config):
     ...

This should behave the same way as:

import cfg_package

@cfn.config(arg=1, configurable_arg=cfg_package.some_config)
def func(arg, configurable_arg):
     ...

but will allow us to avoid duplication of func arguments.