警报类型

不同于像其他一些引导部件的按钮 ，该通知也没有拿出一个 default 或 primary 造型，因为他们是为了提醒以特定的方式给用户。

<div class="alert alert-success" role="alert">
    Some action was completed successfully
</div>
<div class="alert alert-info" role="alert">
    Here is some information. Just FYI.
</div>
<div class="alert alert-warning" role="alert">
    Careful! You're about to do something dangerous.
</div>
<div class="alert alert-danger" role="alert">
    An error (or something dangerous) happened!
</div>