Add Autohide about videogular2

[manekinekko]

I can take this one :)

[Elecash]

👍

[Elecash]

I'm reopening this to change a few things on this feature that maybe I didn't clarify on first description.

Videogular works with a plugin system (modules at the end) that are added and removed by HTML tags. Each plugin to interact with the media elements inside Videogular (video and audio tags inside vg-player tag) need to use an external API (VgAPI).

That means that VgPlayer and VgControls can't know the existence of each other and that's why we can't have references to other plugins (modules) inside vg-player. Like here:
https://github.com/videogular/videogular2/blob/master/src/vg-player/vg-player.ts#L75-L81

To deal with this, I suggest to add a reference to the Videogular element inside VgAPI and each plugin must subscribe to element events individually.

For example:

    ngOnInit() {
      var mouseEnter = Observable.fromEvent(this.api.videogularElement, 'mouseenter');
      mouseEnter.subscribe(this.show.bind(this));

      var mouseLeave = Observable.fromEvent(this.api.videogularElement, 'mouseleave');
      mouseLeave.subscribe(this.hide.bind(this));
    }