title: Limit dropdown options in selectizeInput
body: <p>I have a selectizeInput in my shiny app as follows:</p>
  <pre><code>selectizeInput('foo', 'Select foo:', choices = data, multiple = TRUE)
  </code></pre>
  <p><code>data</code> has hundreds of options and I want to limit what appears in the dropdown.  So, for example, if someone starts a typeahead I don't want the dropdown to show any more than the first five matching items in <code>data</code>.  The reason I want to limit the items in the dropdown for privacy and confidentiality reasons.</p>
  <p>Is there a way of doing this within the <code>selectizeInput</code> options or by editing css or js to achieve it?</p>
  <p>Thanks!</p>