title: Button extension to download all data or only visible data
body: <p>With the button extension to DT package, is there a way to specify that the buttons download either (1) all the data feeding the datatable, or (2) only the data on the visible page.  </p>
  
  <p>Below is the example from the <a href="https://rstudio.github.io/DT/extensions.html" rel="noreferrer">documentation</a>.</p>
  
  <pre><code>datatable(
    iris, extensions = 'Buttons', options = list(
      dom = 'Bfrtip',
      buttons = c('copy', 'csv', 'excel', 'pdf', 'print')
    )
  )
  </code></pre>