vue-loader 13.0.0 on Node.js NPM

Now uses ES modules internally to take advantage of webpack 3 scope hoisting. This should result in smaller bundle sizes.
Now uses PostCSS@6.

The esModule option is now true by default, because this is necessary for ES-module-based scope hoisting to work. This means the export from a *.vue file is now an ES module by default, so async components via dynamic import like this will break:
```
const Foo = () => import('./Foo.vue')
```
Note: the above can continue to work with Vue 2.4 + vue-router 2.7, which will automatically resolve ES modules' default exports when dealing with async components. In earlier versions of Vue and vue-router you will have to do this:
```
const Foo = () => import('./Foo.vue').then(m => m.default)
```
Alternatively, you can turn off the new behavior by explicitly using esModule: false in vue-loader options.

Similarly, old CommonJS-style requires will also need to be updated:
```
// before
const Foo = require('./Foo.vue')

// after
const Foo = require('./Foo.vue').default
```
PostCSS 6 might break old PostCSS plugins that haven't been updated to work with it yet.