[web-animations-1] suggestion: make Animation play method more convenient (add 'time' parameter)

_From @forceuser on July 30, 2015 16:18_

Is it possible to add "time" parameter to Animation.prototype.play(time), it will set starting point of animation and will be shorhand for setting currentTime property.
Now it takes two steps:

```
animation.play();
animation.currentTime = 1200;
```


_Copied from original issue: w3c/web-animations#103_