Add lightning strike trigger

[tesselslate]

Adds a lightning strike trigger, which upon entering, creates a flash of lightning on the screen. There are also additional options:

• Add the rain ambience to the level, as initially requested (toggleable per trigger)
• Create a white flash on the screen when the lightning strikes (toggleable per trigger)
• X offset of the lightning relative to the player
• Delay for the lightning strike after entering the trigger
• The seed used to randomized the nodes/path of the lightning strike

Closes #4

[tesselslate]

Should be better now.