Thunderstorm effect

[tokariew]

Please make thunderstorm effect not being active as default…

I found effect irritating… Some can found these effect hazardous to health…There are people sensitive to sudden changes of brightness…

[C. Blue]

While still enabled by default, it can now be disabled via options.
I listed all "flashy" options in the guide in a new chapter (0.1b) about photosensitivity issues.