Add mix-blend-mode property

[Compositing and Blending Level 1](https://www.w3.org/TR/compositing-1/#mix-blend-mode)

Author: toptalo

src/css/Properties.js

@@ -330,6 +330,7 @@ var Properties = module.exports = {
     "max-width"                         : "<length> | <percentage> | <content-sizing> | none",
     "min-height"                        : "<length> | <percentage> | <content-sizing> | contain-floats | -moz-contain-floats | -webkit-contain-floats",
     "min-width"                         : "<length> | <percentage> | <content-sizing> | contain-floats | -moz-contain-floats | -webkit-contain-floats",
+    "mix-blend-mode"                    : "<blend-mode>",
     "move-to"                           : 1,
 
     // N

src/css/ValidationTypes.js

@@ -329,6 +329,11 @@ copy(ValidationTypes, {
         "<bg-size>":
             "[ <length> | <percentage> | auto ]{1,2} | cover | contain",
 
+        "<blend-mode>":
+            "normal | multiply | screen | overlay | darken | lighten | color-dodge | " +
+            "color-burn | hard-light | soft-light | difference | exclusion | hue | " +
+            "saturation | color | luminosity",
+
         "<border-image-slice>":
         // [<number> | <percentage>]{1,4} && fill?
         // *but* fill can appear between any of the numbers

tests/css/Validation.js

@@ -1114,6 +1114,33 @@ var YUITest = require("yuitest"),
         }
     }));
 
+    suite.add(new ValidationTestCase({
+        property: "mix-blend-mode",
+
+        valid: [
+            "normal",
+            "multiply",
+            "screen",
+            "overlay",
+            "darken",
+            "lighten",
+            "color-dodge",
+            "color-burn",
+            "hard-light",
+            "soft-light",
+            "difference",
+            "exclusion",
+            "hue",
+            "saturation",
+            "color",
+            "luminosity"
+        ],
+
+        invalid: {
+            "foo" : "Expected (normal | multiply | screen | overlay | darken | lighten | color-dodge | color-burn | hard-light | soft-light | difference | exclusion | hue | saturation | color | luminosity) but found 'foo'."
+        }
+    }));
+
     // test <paint>
     suite.add(new ValidationTestCase({
         property: "fill",