title: Unable to filter multiple headers in Custom Container using DT::DataTable
body: <p>I'm using <em>custom container</em> in R ShinyApp. Its currently having Sepal and Petal as headers that are both containing <em>Length and Width columns</em>. So is it possible to get a dropdown from Sepal/Petal for selecting/filtering Length or Width?<br>
  i.e. filter out the headers within headers.
  I'm currently using <em>checkboxGroupInput</em> for this purpose but its not giving required results.
  I have attached my codes as well. Can someone please sort it out. Thanks in advance :)</p>
  
  <pre><code>**MY Codes:**
  library(shiny)
  library(DT)
  
  iris&lt;-iris[,c(5,1:4)]
  
  ui =basicPage(
  tags$head(
  tags$style(type = "text/css",
             HTML("th { text-align: center; }")  )),
  
  selectInput(inputId = "Species", 
            label = "Species:",
            choices = c("All",
                        unique(as.character(iris$Species)))),
  
  checkboxGroupInput(inputId = "columns", label = "Select Variable:",
                   choices =c("Sepal.Length", "Sepal.Width", "Petal.Length", 
   "Petal.Width"),
                   selected = c("Sepal.Length", "Sepal.Width", "Petal.Length", 
   "Petal.Width")),
  
  h2('Iris Table'),
  DT::dataTableOutput('mytable') )
  
  server = function(input, output) {
  output$mytable = DT::renderDataTable({
  
   # a custom table container
  sketch = htmltools::withTags(table(
    class = 'display',
    thead(
      tr(
        th(rowspan = 2, 'Species'),
        th(colspan = 2, 'Sepal'),
        th(colspan = 2, 'Petal')),
      tr(
        lapply(rep(c('Length', 'Width'), 2), th)
      )) )) 
  
    DT::datatable( rownames = FALSE, container = sketch,
                extensions = 'Buttons',
                   options = list(dom = 'Bfrtip',
                               buttons = 
                                 list('colvis', list(
                                   extend = 'collection',
                                   buttons = list(list(extend='csv',
                                                       filename = 'hitStats'),
                                                  list(extend='excel',
                                                       filename = 'hitStats'),
                                                  list(extend='pdf',
                                                       filename= 'hitStats'),
                                                  list(extend='copy',
                                                       filename = 'hitStats'),
                                                  list(extend='print',
                                                filename = 'hitStats')),
  
                                   text = 'Download' ))),
                 {
  
                  data&lt;-iris
  
                  if(input$Species != 'All'){
                    data&lt;-data[data$Species == input$Species,]
                  }    
  
                  data&lt;-data[,c("Species",input$columns),drop=FALSE]   
  
                  data   
                }) })    }
  
  shinyApp(ui = ui, server = server)
  </code></pre>