Darkness pulsing option is not saved

The darkness pulsing option does not get saved to options.txt, and thus reverts to the default value when restarting the game.

How to reproduce:

1. Go to the accessibility menu.
2. Move the "Darkness Pulsing" slider to anything other than 100%.
3. Restart the game.
4. Go back to the accessibility menu and look at the same slider.