This is related to the comment on #368

* added listener for timedMetadata event.

* added callback in RCTVideo for the timed metadata

* exposing onTimedMetadata to JS

* added forgotten method declaration

* returning array of string values

* added metadata type to the array

* added onMetadata method

* overridden onMetadata method on exoplayer2

* added format of return value from onMetadata

* added function reference in README file

Corrected the instructions for running the example

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

* added progressUpdateInterval prop iOS

* add prop to Video.js

* added progressUpdateInterval prop iOS

* add prop to Video.js

* Add Android Expansion File Support

* Added notes for usage with Android Expansion file

* Update README.md

Added notes re: expansion file usage.

* formatting issue fixes

Make visible text *<Video>*  wrapping it with a code block

.default argument for iOS

I could not get the Video component to work by following the provided example. I kept getting an error message saying "React.createElement: type should not be null, undefined, boolean, or number.". When I changed the way I include the component, it started working. It might be related to recent versions of react-native, not sure. You're welcome to modify the suggested text if you like. Thanks for a great piece of work!