不支持 webpack module.rules 的 loader 缩写

使用
```
rules: [{
loader: 'vue-loader'
}]
```
这样的webpack config语法会有问题。

Reproduce the problem with the shown webpack config using a rule that contains only loader: 'vue-loader'. Trace how vux-loader handles module.rules and compare this shorthand with the supported syntax; done means the configuration works without the reported problem.

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