Add support for Twitch player variables.

[casperOne]

Version: 1.2.1

From the documentation it seems that Twitch player variables are not supported yet:

https://discuss.dev.twitch.tv/t/embedded-player-set-start-time/7148

Specifically, I'm looking to set the start time of the video (a la YouTube). I imagine it would look something like this:

<ReactPlayer
  url="https://www.twitch.tv/videos/106400740"
  config={{
    twitch: {
      playerVars: {
        time: '30s'
      }
    }
  }}
/>

Note, I've tried that here, to no effect:

https://codesandbox.io/s/kopnz61ly7