Add a way to get nested dicts out of a ConfigDict for libraries that need them (e.g. logging.config, beaker)

@rcarmo said in #772

May I suggest ensuring that the Python logging configuration becomes part of the overall config?

The bottle config is flat (i.e. keys.with.dots.in.them) but the logging.config module expects a nested structure. Some other frameworks need that, too (e.g. beaker). We have ConfigDict.load_dict() to flatten a nested dictionary or json. The reverse would be required to easily populate logging configuration out of bottle.