feat(Windows): Adds Windows support to react-native-video

This PR adds react-native-windows support to react-native-video.  The Video component is implemented with a XAML MediaPlayerElement.  Most of the features implemented by Android (and some additional ones) are implemented by Windows.

Known issues and missing features include:
* onReadyForDisplay event
* local URI convention (e.g., "broadchurch" in examples changed to require("./broadchurch.mp4")
* `playableDuration` in `onVideoProgress` event is always 0.0
* `playInBackground` is not yet supported
* Volume settings are applied, but the UWP control does not handle it properly

example/windows/VideoPlayer/project.json

@@ -0,0 +1,17 @@
+{
+  "dependencies": {
+    "Facebook.CSSLayout": "2.0.1-pre",
+    "Microsoft.NETCore.UniversalWindowsPlatform": "5.2.2"
+  },
+  "frameworks": {
+    "uap10.0": {}
+  },
+  "runtimes": {
+    "win10-arm": {},
+    "win10-arm-aot": {},
+    "win10-x86": {},
+    "win10-x86-aot": {},
+    "win10-x64": {},
+    "win10-x64-aot": {}
+  }
+}