Send message to all users in `category`.

def message(self, category, subject, msg_file):
        """Send message to all users in `category`."""
        users = getattr(self.sub, category)
        if not users:
            print('There are no {} users on {}.'.format(category, self.sub))
            return

        if msg_file:
            try:
                msg = open(msg_file).read()
            except IOError as error:
                print(str(error))
                return
        else:
            print('Enter message:')
            msg = sys.stdin.read()

        print('You are about to send the following message to the users {}:'
              .format(', '.join([str(x) for x in users])))
        print('---BEGIN MESSAGE---\n{}\n---END MESSAGE---'.format(msg))
        if input('Are you sure? yes/[no]: ').lower() not in ['y', 'yes']:
            print('Message sending aborted.')
            return
        for user in users:
            user.send_message(subject, msg)
            print('Sent to: {}'.format(user))