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/YogaKitSample/YogaKitSample/ViewControllers/BasicViewController.swift

@@ -17,8 +17,8 @@ final class BasicViewController: UIViewController {
         root.backgroundColor = .white
         root.configureLayout { (layout) in
             layout.isEnabled = true
-            layout.width = containerSize.width
-            layout.height = containerSize.height
+            layout.width = YGValue(containerSize.width)
+            layout.height = YGValue(containerSize.height)
             layout.alignItems = .center
             layout.justifyContent = .center
         }