[laravel-mix] Fix /deep/ in css of vue

laravel-mix deep css

https://github.com/JeffreyWay/laravel-mix/issues/1850#issuecomment-528527499

Hi everyone, here is the solution:

```js
mix.options({
    postCss: [
        postcss.plugin('deepCssVueScoped', () => {
            let sign = '[deep]';
            return function (root, result) {
                root.walkRules((rule) => {
                    if (~rule.selector.indexOf(sign)) {
                        rule.selector = rule.selector.replace(sign, '/deep/');
                    }
                });
            };
        })
    ]
});
```

And use it as follows: 

```scss
.auth {
    display: flex;
    [deep] .auth-container {
        position: relative;
        .auth-title {
            .logo {
                height: 100px;
                width: 100px;
            }
        }
    }
}
```