color: add contrast method

Author: etpinard

src/components/color/index.js

@@ -57,6 +57,16 @@ color.combine = function(front, back) {
     return tinycolor(fcflat).toRgbString();
 };
 
+color.contrast = function(cstr, lightAmount, darkAmount) {
+    var tc = tinycolor(cstr);
+
+    var newColor = tc.isLight() ?
+        tc.darken(darkAmount) :
+        tc.lighten(lightAmount);
+
+    return newColor.toString();
+};
+
 color.stroke = function(s, c) {
     var tc = tinycolor(c);
     s.style({'stroke': color.tinyRGB(tc), 'stroke-opacity': tc.getAlpha()});