title: add custom Class to alert dialog in bootbox bootstrap jquery plugin
body: <p>I am using <a href="http://bootboxjs.com/" rel="noreferrer">Bootbox jquery plugin</a> to show modals.</p>
  
  <p>this is a simple usage of that to show alert dialogs :</p>
  
  <pre><code>bootbox.alert({
       title: 'Message Title',
       message: "this is message Body"
  });
  </code></pre>
  
  <p>This plugin has a <code>className</code> option to add custom Classes to apply to the dialog wrapper created by <code>dialog()</code> method but we can not use it for <code>alert()</code> method.</p>
  
  <p>a way to add custom classes to <code>alert()</code> is use <code>dialog()</code> to create those. but in this case we must create buttons like <strong>OK</strong> (or <strong>Cancel</strong> in<code>prompt()</code> method) manually every time that we want an simple Alert that is Is time consuming, like this:</p>
  
  <pre><code>bootbox.dialog({
    message: "I am a custom dialog",
    title: "Custom title",
    buttons: {
      success: {
        label: "Success!",
        className: "btn-success",
        callback: function() {
          Example.show("great success");
        }
      }
  });
  </code></pre>
  
  <p>is there any way to add custom classes to alert boxes via <code>alert()</code> method?</p>