Make it work with vue-play

[egoist]

To make it work with vue-play, we need to change the webpack config in config.js to something like:

module.exports = {
  entry: {
    app: './play/app.js',
    preview: './play/preview.js'
  },
  // add hmr client to these entries
  hmrEntries: ['app', 'preview'],
  // produce html files for these entries
  html: [
    {title: 'Vue-Play Panel', chunks: ['app']},
    {title: 'Vue-Play Preview', chunks: ['preview'], filename: 'preview.html'}
  ]
}

And this PR allows:

• options.entry to be undefined if using --config
• options.html can be an array
• use options.hmrEntries to add hmr client