Add random greyscale color feature

[jamjar919]

This pull req adds a new feature - The ability to have the script generate random greyscale colors for the strokes. Here's a demo.

The behaviour is controlled with the randomStrokeColor flag in options, by default set to false. When the flag is set to true, each particle is assigned a random color, picked from the range #000 - #FFF. The range of colors can be selected by setting the strokeColorMin and strokeColorMax options to numbers between 0 and 15, where 0 = #000 and 15 = #FFF.

I've also updated the readme to include more information but haven't updated the min file as I am unsure how you have split the code up.