CarouselController has no getter for current index

[MilesAdamson]

Unlike flutters TabController, CarouselController has no getter for the current index. This makes passing a controller into a widget which has a CarouselSlider harder to use.

Since this is a pattern used by flutter for their tab controllers I think it could be good to add.

[RaulUrtecho]

Also not very clear the documentation on the controller dispose, is it automatic?

[leeflix]

When will this be implemented? @serenader2014