Fog support, Draw distance cutoff

Robert, I've added support for fog so that the lights shafts fade into it nicely. I also added a draw distance parameter to stop drawing the effect beyond a certain distance as a performance optimization. Tried to keep the changes as minimal as possible. Feel free to use any parts of it if you feel like it's useful for you :)

Log:
* Support fading into fog (all Unity fog modes supported, using shader keywords to optimize shader runtime)
* Added 'Draw distance' parameter to stop the effect from rendering after a certain distance from the camera. Hardcoded fade out at the final 10% of the range for now.