I see the way it's done in cli.py
but it's not set up as a utility for plugin code to make use of.
From http://errbot.io/en/latest/user_guide/plugin_development/configuration.html :
Errbot can keep a simple python object for the configuration of your plugin. This avoids the need for admins to configure settings in some kind of configuration file, instead allowing configuration to happen directly through chat commands.
But, I like config files! I prefer to use them for some things if possible.