How to include a "select all" option in a CheckboxGroupInput

Using the code below, I could create a very nice list of choices. I would like to add a "select all" choice right above the first choice (Ohio). Is there any easy fix for this?

Thanks,

Nader

collapsibleCheckboxGroupInput(
  inputId = "county_", "Select County:", i = 5, selected = "Ohio",
  choices = c("Ohio", sort(unique((map_Ohio$County[map_Ohio$County !="Ohio"]))))),

 collapsibleCheckboxGroupInput <- 
  function(inputId, label, i, choices = NULL, selected = NULL, width = NULL, 
           choiceNames = NULL, choiceValues = NULL){
    input <- checkboxGroupInput(inputId, label, choices = choices, 
                                selected = selected, width = width,
                                choiceNames = choiceNames, 
                                choiceValues = choiceValues)
    checkboxes <- input[[3]][[2]][[3]][[1]]
    id_btn <- paste0(inputId, "_btn")
    id_div <- paste0(inputId, "_collapsible")
    btn <- actionButton(id_btn, "More...", 
                        icon = icon("collapse-up", lib = "glyphicon"), 
                        class = "btn-primary btn-sm", 
                        `data-toggle`="collapse", 
                        `data-target` = paste0("#", id_div))
    collapsible <- div(id = id_div, class = "collapse")
    collapsible$children <- checkboxes[(i+1):length(checkboxes)]
    children <- c(checkboxes[1:i], list(btn), list(collapsible))
    input[[3]][[2]][[3]][[1]] <- children
    script <- sprintf('$(document).ready(function(){
                      $("#%s_collapsible").on("hide.bs.collapse", function(){
                      $("#%s_btn").html("<span class=\\\"glyphicon glyphicon-collapse-down\\\"></span> More...");
                      });
                      $("#%s_collapsible").on("show.bs.collapse", function(){
                      $("#%s_btn").html("<span class=\\\"glyphicon glyphicon-collapse-up\\\"></span> Less...");
                      });
  });', inputId, inputId, inputId, inputId)
    tagList(input, tags$script(HTML(script)))
    }