Added support for width & height for text images

Summary:
public

Previously, `<Image>` elements embedded inside `<Text>` ignored all style attributes and props apart from `source`. Now, the `width`, `height` and `resizeMode` styles are observed. I've also added a transparent placeholder to be displayed while the image is loading, to prevent the layout from changing after the image has loaded.

Reviewed By: javache

Differential Revision: D2838659

fb-gh-sync-id: c27f9685b6976705ac2b24075922b2bf247e06ba

Libraries/Image/RCTShadowVirtualImage.h

@@ -10,6 +10,7 @@
 #import "RCTShadowView.h"
 #import "RCTImageComponent.h"
 #import "RCTImageSource.h"
+#import "RCTResizeMode.h"
 
 @class RCTBridge;
 
@@ -22,5 +23,6 @@
 - (instancetype)initWithBridge:(RCTBridge *)bridge;
 
 @property (nonatomic, strong) RCTImageSource *source;
+@property (nonatomic, assign) RCTResizeMode resizeMode;
 
 @end