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/SwiftViewController.swift

@@ -12,6 +12,11 @@ import YogaKit
 
 struct DemoItem {
     let name: String
+    root.backgroundColor = .red
+    root.yoga.isEnabled = true
+    root.yoga.width = YGValue(self.view.bounds.size.width)
+    root.yoga.height = YGValue(self.view.bounds.size.height)
+    root.yoga.alignItems = .center
 }
 
 final class SwiftViewController: UIViewController, IGListAdapterDataSource {