divider and divider-vertical variables needed.

Created by: MechanisM

Please add divider and divider-vertical into variables.less and into "Customize variables" section in docs page. So people can customize divider colors in case if dropdown background color is modified to dark, then divider can look ugly when they're light(its default). So if we can change background color for dropdown(we have it in variables), then divider colors must be changed too(should be added into variables too).