zoom specific layers

[mtennekes]

For interactive maps, we would like to have layers that are zoom-specific, e.g.

tm_shape(NLD_prov) +
  tm_polygons(zoom = c(5, 7)) +
tm_shape(NLD_muni) +
 tm_polygons(zoom = c(7, 10))

[bvarick]

I'd love to see this too!