Implemented percentage values in YogaKit

Summary:
We still need to wait for the `YGUnitPoint` PR to be merged :) But please let me know what you think. One caveat: because of a limitation of Swift, a literal value can be automatically understood as a point-based `YGValue`, but variables have to be explicitly cast. I haven't found a way around it yet:

```
view.yoga.width = 10 // value == 10, unit == YGUnitPixel

let a: CGFloat = 100
view.yoga.height = a // Compiler error
view.yoga.height = YGValue(a) // works, not great
```
Closes https://github.com/facebook/yoga/pull/390

Reviewed By: emilsjolander

Differential Revision: D4954021

Pulled By: maxoll

fbshipit-source-id: 5eff6aeb6dd969d0d5dc557b149bb5819b0e31de

YogaKit/Source/YGLayoutExtensions.swift

@@ -0,0 +1,46 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+postfix operator %
+
+extension Int {
+    public static postfix func %(value: Int) -> YGValue {
+        return YGValue(value: Float(value), unit: .percent)
+    }
+}
+
+extension Float {
+    public static postfix func %(value: Float) -> YGValue {
+        return YGValue(value: value, unit: .percent)
+    }
+}
+
+extension CGFloat {
+    public static postfix func %(value: CGFloat) -> YGValue {
+        return YGValue(value: Float(value), unit: .percent)
+    }
+}
+
+extension YGValue : ExpressibleByIntegerLiteral, ExpressibleByFloatLiteral {
+    public init(integerLiteral value: Int) {
+        self = YGValue(value: Float(value), unit: .point)
+    }
+	
+    public init(floatLiteral value: Float) {
+        self = YGValue(value: value, unit: .point)
+    }
+  
+    public init(_ value: Float) {
+        self = YGValue(value: value, unit: .point)
+    }
+  
+    public init(_ value: CGFloat) {
+        self = YGValue(value: Float(value), unit: .point)
+    }
+}