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/LayoutInclusionViewController.swift

@@ -29,7 +29,7 @@ final class LayoutInclusionViewController: UIViewController {
         contentView.configureLayout { (layout) in
             layout.isEnabled = true
             layout.height = 300
-            layout.width = self.view.bounds.size.width
+            layout.width = YGValue(self.view.bounds.size.width)
             layout.flexDirection = .row
             layout.justifyContent = .center
             layout.paddingHorizontal = 25