按钮下拉菜单
结合使用.btn-group(btn-group-lg/btn-group-sm/btn-group-xs) .dropup .btn .dropdown-menu 按钮下拉菜单是在普通的下拉菜单的基础上封装了.btn样式得效果，就类似于单击一个button按钮，然后显示隐藏的下拉菜单。
底部100%的宽度，即靠着上边框，向上方向弹出
margin-bottom: 2px;
}
<Meta charset="utf-8">
<Meta http-equiv="X-UA-Compatible" content="IE=edge">
<Meta name="viewport" content="width=device-width,initial-scale=1">
Bootstrap 101 Template
<link href="bootstrap-3.3.7-dist/css/bootstrap.min.css" rel="stylesheet">
<div class="btn-group">
<button type="button" class="btn btn-success dropdown-toggle" data-toggle="dropdown">
Success <span class="caret">
<ul class="dropdown-menu">
<div class="btn-group">
<button type="button" class="btn btn-success">Button
<button type="button" class="btn btn-success dropdown-toggle" data-toggle="dropdown">
<span class="caret">