This would it more convenient to extend chains for local contexts.  Currently, we write:

    local = parent.new_child(dict(foreground='white', background='cyan'))

Instead, it would be easier to write:

    local = parent.new_child(foreground='white', background='cyan')

The new code would look like this:

    def new_child(self, m=None, **kwargs):
        '''New ChainMap with a new map followed by all previous maps.
        If no map is provided, an empty dict is used.
        Keyword arguments update the map or new empty dict:
        '''
        if m is None:
            m = kwargs
        elif kwargs:
            m.update(kwargs)
        return self.__class__(m, *self.maps)