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