updated sandwich layout
updated reset & sandwich layout

1. p tag in _reset.scss
2. max-width in _sandwich.scss
updated style rules
updated reset & mixin

1. styles of <p> and <code>
2. mixins: 'block_content', 'triangle'
updated <pre> and <code>
改变「三文治布局」的激活方式

data-layout="sandwich" => class="layout--sandwich"
调整 <pre> 和 <code> 的样式

1. padding
2. margin
3. background color
4. border
更新 CHANGELOG.md

添加代码相关标签的描述