more informative navigator in html animation

[yihui]

instead of these green dots, we may use text labels directly (e.g. country names), which can be very useful to navigate through different image frames

@brentertz brentertz/scianimator#5

[barryrowlingson]

There's a nice animator in the choroplethr package that uses a range slider so you can grab and drag to animate. They use it to make animated space-time maps, but its a general solution and would be nice to have in the animation package. It uses AngularJS so its totally modern...