Added onTimedMetadata callback for iOS player and Exoplayer (#487)

* 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