Add animated dash line example

Something similar to 


![ezgif com-video-to-gif 2](https://user-images.githubusercontent.com/4394910/43017530-4b76abe8-8c0b-11e8-8e45-3cf630f2a21f.gif)



Related to #688 and https://github.com/mapbox/mapbox-plugins-android/issues/484

Other helpful/related examples:

http://jsbin.com/lokimezuyo/edit?html,output

https://www.mapbox.com/labs/elementum/