Introduce luminance function.

The behaviour of this function is identical to luma prior to LESS 1.7.

lib/less/functions.js

@@ -95,6 +95,14 @@ tree.functions = {
     luma: function (color) {
         return new(tree.Dimension)(Math.round(color.luma() * color.alpha * 100), '%');
     },
+    luminance: function (color) {
+        var luminance =
+            (0.2126 * color.rgb[0] / 255)
+          + (0.7152 * color.rgb[1] / 255)
+          + (0.0722 * color.rgb[2] / 255);
+
+        return new(tree.Dimension)(Math.round(luminance * color.alpha * 100), '%');
+    },
     saturate: function (color, amount) {
         // filter: saturate(3.2);
         // should be kept as is, so check for color