Added yiq-is-light function, refactored yiq-contrast-color to take advantage

It'd be really nice if we could determine if the color is light or not without needing to call `yiq-contrast-color` and comparing it to the color variable names. So, in that vein, I added a `yiq-is-light` function that simply returns `true` or `false` based on color and threshold input. Also refactored `yiq-contrast-color` to take advantage of the new function.