Register a flag of a parent command

        :Parameters:
            - `optionName`: String. Name of option
            - `value`: Mixed. Value of parsed flag`

def registerParentFlag(self, optionName, value):
        '''Register a flag of a parent command

        :Parameters:
            - `optionName`: String. Name of option
            - `value`: Mixed. Value of parsed flag`
        '''
        self.parentFlags.update({optionName: value})
        return self