title: checkboxGroupInput with checkboxInput behaviour
body: <p>I have a checkboxGroupInput, but I want to have checkboxInput behaviour. I want to have independent reactive functions, that are only activated when you (un)tick <code>a</code> or <code>b</code>. This is easily possible with seperate <code>checkboxInput</code>s, but not within a <code>checkboxGroupInput</code>.</p>
  
  <pre><code>ui &lt;- shinyUI(pageWithSidebar(
      headerPanel("checkboxGroupInput"),
      sidebarPanel(
          checkboxGroupInput('sources',
                             label='Sources',
                             choices=list('a'='a',
                                          'b'='b')),
          checkboxInput('a',
                        'a'),
          checkboxInput('b',
                        'b')
      ),
      mainPanel(
          # empty
      )
  ))
  
  server &lt;- shinyServer(function(input, output) {
      thingy &lt;- reactive({
          return('a' %in% input$sources)
      })
      observeEvent(thingy(), {
          print("I'm here (a+b)")
      })
      observeEvent(input$a, {
          print("I'm here (a only)")
      })
      observeEvent(input$b, {
          print("I'm here (b only)")
      })
  
  })
  
  shinyApp(ui=ui,server=server) 
  </code></pre>
  
  <p>What I tried in the above example is store the boolean whether <code>a</code> is in the <code>checkboxGroupInput</code>. Even if the value stays <code>TRUE</code> (i.e., clicking <code>b</code> repeatedly), <code>thingy()</code> is still getting activated.  </p>