Support parent container for scroll behavior

[erguotou520]

Feature request?

VueRouter use window.scrollTo to do scroll, but in some case, the scroll container may be not the body, is that possible to support custom scroll parent like:

scrollBehavior () {
  return { container: 'container-selector', x: 0, y: 0 }
}

For realization, update window.scrollTo to $container.scrollTop = position.y; $container.scrollLeft = position.x

[posva]

dup of #1187