refactored custom transitions api

lib/ios/RNNTransitionStateHolder.m

 	self.startAlpha = [RNNUtils getDoubleOrKey:transition withKey:@"startAlpha" withDefault:1];
 	self.endAlpha = [RNNUtils getDoubleOrKey:transition withKey:@"endAlpha" withDefault:1];
 	self.interactivePop = [RNNUtils getBoolOrKey:transition withKey:@"interactivePop" withDefault:NO];
-	self.startX = [RNNUtils getDoubleOrKey:transition withKey:@"startX" withDefault:0];
-	self.startY = [RNNUtils getDoubleOrKey:transition withKey:@"startY" withDefault:0];
-	self.endX = [RNNUtils getDoubleOrKey:transition withKey:@"endX" withDefault:0];
-	self.endY = [RNNUtils getDoubleOrKey:transition withKey:@"endY" withDefault:0];
+	self.startX = [RNNUtils getDoubleOrKey:transition[@"x"] withKey:@"from" withDefault:0];
+	self.startY = [RNNUtils getDoubleOrKey:transition[@"y"] withKey:@"from" withDefault:0];
+	self.endX = [RNNUtils getDoubleOrKey:transition[@"x"] withKey:@"to" withDefault:0];
+	self.endY = [RNNUtils getDoubleOrKey:transition[@"y"] withKey:@"to" withDefault:0];
 	self.fromId = [transition objectForKey:@"fromId"];
 	self.toId = [transition objectForKey:@"toId"];
 	self.fromElement = nil;

playground/src/screens/CustomTransitionDestination.js

   pop() {
     Navigation.pop(this.props.componentId, {
       customTransition: {
-        animations: [{ type: 'sharedElement', fromId: 'title2', toId: 'title1', startDelay: 0, springVelocity: 0.2, duration: 0.5 },
+        animations: [
+          { type: 'sharedElement', fromId: 'title2', toId: 'title1', startDelay: 0, springVelocity: 0.2, duration: 0.5 },
         { type: 'sharedElement', toId: 'image1', fromId: 'customDestinationImage', startDelay: 0, springVelocity: 0.2, duration: 0.5 },
         { type: 'sharedElement', toId: 'image2', fromId: 'customDestinationImage2', startDelay: 0, duration: 0.8 },
-        { fromId: 'image4', startY: 50, startX: 50, startAlpha: 0, startDelay: 0, duration: 0.8, springVelocity: 0.5 },
-        { fromId: 'customDestinationParagraph', endY: 50, endX: 50, endAlpha: 0, startAlpha: 1, startDelay: 0, duration: 0.8 }
+        { fromId: 'image4', x: { from: 50}, y: { from: 50}, startAlpha: 0, startDelay: 0, duration: 0.8, springVelocity: 0.5 },
+        { fromId: 'customDestinationParagraph', y: { to: 50}, x: { to: 50}, endAlpha: 0, startAlpha: 1, startDelay: 0, duration: 0.8 }
         ],
         duration: 0.8
       }

playground/src/screens/CustomTransitionOrigin.js

               { type: 'sharedElement', fromId: 'image1', toId: 'customDestinationImage', startDelay: 0, springVelocity: 0.9,
               springDamping: 0.9, duration: 0.8, interactivePop: true },
               { type: 'sharedElement', fromId: 'image2', toId: 'customDestinationImage2', startDelay: 0, duration: 0.8 },
-              { fromId: 'image4', endY: 50, endX: 50, endAlpha: 0, startDelay: 0, duration: 0.8, springVelocity: 0.5 },
+              { fromId: 'image4', x: { to: 50 }, y: { to: 50 }, endAlpha: 0, startDelay: 0, duration: 0.8, springVelocity: 0.5 },
               { fromId: 'customDestinationParagraph', startY: 50, startAlpha: 0, endAlpha: 1, startDelay: 0, duration: 0.8 }
             ],
             duration: 0.8