Add minicharts to group layers

[kstierhoff]

Hi, Thank you for this package, it's extremely useful!

Is it possible/difficult to add to addMinicharts() the ability to add the minicharts to group layers, such as:

leaflet::addCircleMarkers(map, lng = NULL, lat = NULL, radius = 10,
  layerId = NULL, group = "group_name")

So that the charts may be turned on/off via:

addLayersControl(
    overlayGroups = c("group_name"))