1. Loaded event will give the information about available video tracks
2. selectedVideoTrack prop will give the interface to select the video track. This prop will take the height of the video (Similar to youtube like 144p, 350p etc) and set the video track. If height is 0 then all video tracks will be added to list so that it will work like 'Auto' in youtube
* 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
