title: RShiny-CSS reactive message
body: <p>I'm using Shiny for a while and recently started customize my app with CSS which is pretty cool. I managed to show a message box when the pointer goes over a particular element but now I would like to show a variable. For example in the following app when the mouse goes on the slider button, inside the message box I would like to compute the slider value i.e "input$test" : </p>
  
  <pre><code>#### ui.R
  
  shinyUI(fluidPage(
  tags$head(tags$style(HTML('
  
                          .irs.js-irs-0.irs-with-grid .irs-slider.single:hover:after {
                          content: "SLIDER VALUE HERE";
                          background: #333;
                          background: rgba(0,0,0,.8);
                          border-radius: 5px;
                          bottom: 26px;
                          color: #fff;
                          left: 20%;
                          padding: 5px 15px;
                          position: absolute;
                          width: 220px;
                          text-align: center;
                          }
  
                          '))),
  
  sliderInput("test", "TEST:", 
            min=0, max=1000, value=500)
  ))
  
  #### server.R
  
  library(shiny)
  shinyServer(function(input, output) {
  })
  </code></pre>
  
  <p>Thanks for your help.</p>