灰化列表元素只能灰化第一个

### Description

```html

• 1


• 1

.item {
background: 一个base64图
}

```
上述情况下，如果指定grayBlock: ['.item'],
那么变成灰色的只有第一个

### Steps to reproduce

如上所述

**Expected behavior:** [What you expected to happen]

**Actual behavior:** [What actually happened]

**Link to an example:** [If you're reporting a bug that's not reproducible, please try to reproduce it on [JSFiddle](https://jsfiddle.net/), [JS Bin](https://jsbin.com), [CodePen](http://codepen.io/) or a similar service and paste a link here]

### Plugin configration

for example:

```javascript
new SkeletonPlugin({
pathname: path.resolve(__dirname, `./src`),
port: '7890',
loading: 'spin',
svg: {
color: '#EFEFEF',
shape: 'circle',
shapeOpposite: ['.red']
},
image: {
shape: 'rect', // `rect` | `circle`
color: '#EFEFEF',
shapeOpposite: ['.white']
},
pseudo: {
color: '#EFEFEF', // or transparent
shape: 'circle', // circle | rect
shapeOpposite: ['.apple', '.pen']
},
button: {
color: '#EFEFEF',
excludes: ['.center a']
},
defer: 5000,
excludes: [],
remove: [],
hide: ['.ag-text', '.ag-image'],
grayBlock: ['#header'],
cssUnit: 'vw',
cookies: [{
name: 'SID',
value: 'xxxxxx',
url: 'https://xx.xxx.xx'
}, {
name: 'USERID',
value: 'xxxxxx',
url: 'https://xx.xxx.xx'
}]
})
```

### Versions

- Page Skeleton:
- Webpack: