Add text-underline-position-property to fit current CSS Text Decoration Module Level 3 [CR]

[CSS Text Decoration Module Level 3](https://www.w3.org/TR/css-text-decor-3/#text-underline-position-property)

Author: toptalo

src/css/Properties.js

@@ -437,6 +437,7 @@ var Properties = module.exports = {
     "text-decoration-style"             : "<text-decoration-style>",
     "text-decoration-skip"              : "none | [ objects || spaces || ink || edges || box-decoration ]",
     "-webkit-text-decoration-skip"      : "none | [ objects || spaces || ink || edges || box-decoration ]",
+    "text-underline-position"           : "auto | [ under || [ left | right ] ]",
     "text-emphasis"                     : 1,
     "text-height"                       : 1,
     "text-indent"                       : "<length> | <percentage>",

tests/css/Validation.js

@@ -1518,14 +1518,32 @@ var YUITest = require("yuitest"),
             "edges",
             "box-decoration",
             "objects spaces ink"
-    ],
+        ],
 
         invalid: {
             "none objects" : "Expected end of value but found 'objects'.",
             "foo" : "Expected (none | [ objects || spaces || ink || edges || box-decoration ]) but found 'foo'."
         }
     }));
 
+    suite.add(new ValidationTestCase({
+        property: "text-underline-position",
+
+        valid: [
+            "auto",
+            "under",
+            "left",
+            "right",
+            "under left"
+        ],
+
+        invalid: {
+            "auto under" : "Expected end of value but found 'under'.",
+            "left right" : "Expected end of value but found 'right'.",
+            "foo" : "Expected (auto | [ under || [ left | right ] ]) but found 'foo'."
+        }
+    }));
+
     suite.add(new ValidationTestCase({
         property: "text-rendering",