Icons should be transpiled beforehand

Reproduce:
- Run `vue init webpack test`
- Add vue-octicon & an icon
- Run `npm run build`
- Watch UglifyJs fail

Workaround: Using this webpack configuration
```js
    {
        test: /\.js$/,
        loader: 'babel-loader',
        include: [resolve('src'), resolve('test'), resolve('node_modules/vue-octicon')]
      },
```