def value(key, context)
      overrides_for(context).each { |conf| return conf[key] if conf.key?(key) }
      options.fetch(key)
    end
def value(key, context)
      overrides_for(context).each { |conf| return conf[key] if conf.key?(key) }
      options.fetch(key)
    end
Retrieves the value, if any, for the given +key+ in the given +context+.

 Raises an error if neither the context nor this config have a value for
 the key.