### Flex
由于Flex 比较难写尴尬的兼容，所以这里我们也把它提了出来，使用autoprefixer的小伙伴可以无视这段。
- 初始化 flex
```css
.flex
```
- 水平排列 justify-content
```css
.flex-justify-center
.flex-justify-start
.flex-justify-end
.flex-justify-space-between
```
- 垂直排列 align-items
```css
.flex-align-start
.flex-align-end
.flex-align-center
.flex-align-stretch
.flex-align-baseline
```
- 纵向排列
```css
.flex-column
```
- 强制换行
```css
.flex-wrap
```
- flex
```
.flex-1
.flex-2
.flex-3
```
- grow
```css
.flex-grow-1
.flex-grow-2
.flex-grow-3
```
