Add contrast function following the same format as sass. Added tests.

Added luma calculation.

lib/less/functions.js

@@ -44,6 +44,12 @@ tree.functions = {
     alpha: function (color) {
         return new(tree.Dimension)(color.toHSL().a);
     },
+    luma: function (color) {
+        return new(tree.Dimension)(Math.round((0.2126 * (color.rgb[0]/255) +
+            0.7152 * (color.rgb[1]/255) +
+            0.0722 * (color.rgb[2]/255))
+            * color.alpha * 100), '%');
+    },
     saturate: function (color, amount) {
         var hsl = color.toHSL();
 
@@ -127,6 +133,24 @@ tree.functions = {
     greyscale: function (color) {
         return this.desaturate(color, new(tree.Dimension)(100));
     },
+    contrast: function (color, dark, light, threshold) {
+        if (typeof light === 'undefined') {
+            light = this.rgba(255, 255, 255, 1.0);
+        }
+        if (typeof dark === 'undefined') {
+            dark = this.rgba(0, 0, 0, 1.0);
+        }
+        if (typeof threshold === 'undefined') {
+            threshold = 0.43;
+        } else {
+            threshold = threshold.value;
+        }
+        if (((0.2126 * (color.rgb[0]/255) + 0.7152 * (color.rgb[1]/255) + 0.0722 * (color.rgb[2]/255)) * color.alpha) < threshold) {
+            return light;
+        } else {
+            return dark;
+        }
+    },
     e: function (str) {
         return new(tree.Anonymous)(str instanceof tree.JavaScript ? str.evaluated : str);
     },