title: How to enable overflow for `react-bootstrap` NavDropdown
body: <p>I'm using <code>react-bootstrap</code>'s NavDropdown as part of a NavBar to display a list of data. Occasionally the list is longer than the rest of the view, cutting off the bottom of the list. I need to be able to add <code>overflow: auto</code> to the list, but can't figure out how to do it. Wrapping the NavDropdown or the items within it in a styled div breaks <code>react-bootstrap</code>'s styling. Attempting to style the NavDropdown directly (via <code>className</code> or <code>style</code>) breaks <code>react-bootstrap</code>'s styling. Anyone been able to solve this?</p>
  
  <p>The code is roughly:</p>
  
  <pre><code>&lt;NavBar&gt;
    &lt;NavBar.Collapse&gt;
      &lt;Nav&gt;
        &lt;NavDropdown&gt;
          lots of &lt;MenuItem /&gt;
        &lt;/NavDropdown&gt;
      &lt;/Nav&gt;
    &lt;/NavBar.Collapse&gt;
  &lt;/NavBar&gt;
  </code></pre>