title: Bootstrap 4 input group RTL issue
body: <p>I have a requirement to make a bootstrap v4 page RTL.</p>
  
  <p>As I add the <code>dir="rtl"</code> attribute, the <code>input-group</code> element styles are messed up: </p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;link href="https://v4-alpha.getbootstrap.com/dist/css/bootstrap.min.css" rel="stylesheet"/&gt;
  
  &lt;div class="container" dir="rtl"&gt;
    &lt;div class="row"&gt;
      &lt;div class="col"&gt;
        &lt;div class="input-group"&gt;
        &lt;span class="input-group-addon" id="basic-addon1"&gt;@&lt;/span&gt;
        &lt;input type="text" class="form-control" placeholder="Username" aria-describedby="basic-addon1"&gt;
      &lt;/div&gt;
      &lt;br&gt;
      &lt;div class="input-group"&gt;
        &lt;input type="text" class="form-control" placeholder="Recipient's username" aria-describedby="basic-addon2"&gt;
        &lt;span class="input-group-addon" id="basic-addon2"&gt;@example.com&lt;/span&gt;
      &lt;/div&gt;
      &lt;br&gt;
      &lt;label for="basic-url"&gt;Your vanity URL&lt;/label&gt;
      &lt;div class="input-group"&gt;
        &lt;span class="input-group-addon" id="basic-addon3"&gt;https://example.com/users/&lt;/span&gt;
        &lt;input type="text" class="form-control" id="basic-url" aria-describedby="basic-addon3"&gt;
      &lt;/div&gt;
      &lt;br&gt;
      &lt;div class="input-group"&gt;
        &lt;span class="input-group-addon"&gt;$&lt;/span&gt;
        &lt;input type="text" class="form-control" aria-label="Amount (to the nearest dollar)"&gt;
        &lt;span class="input-group-addon"&gt;.00&lt;/span&gt;
      &lt;/div&gt;
      &lt;br&gt;
      &lt;div class="input-group"&gt;
        &lt;span class="input-group-addon"&gt;$&lt;/span&gt;
        &lt;span class="input-group-addon"&gt;0.00&lt;/span&gt;
        &lt;input type="text" class="form-control" aria-label="Amount (to the nearest dollar)"&gt;
      &lt;/div&gt;
      &lt;/div&gt;
    &lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>I'm using <em>Bootstrap v4.0.0-alpha.6</em></p>
  
  <p>I'm looking for a way to fix those styles?</p>