constrast function modified to use luma instead of lightness and default...

... threshold changed to 43%

I found that dotless was not giving the same output than less.js when using the contrast function. After some investigation I noticed less.js uses the color luma (and a different threshold) while dotless directly uses the color brightness. The luma is a better measure of the perceived brightness, so I changed the contrast implementation.

Also, I did not found any existing implementation of the luma function in dotless, I could try to implement it for a future pull request, and refactor this code to use it.