Allow postcss options to be a function (webpack) => PostCSSPlugin[]

To allow the `postcss-import` add dependencies to webpack, it should accept some options like:

``` js
{
  addDependencyTo: webpack
}
```

So the `vue-loader` should also allow the postcss options be a `function(webpack): []`, just like [postcss-loader](https://github.com/postcss/postcss-loader#integration-with-postcss-import).
