feat(cli-service): add history api fallback for multi-page mode

[jkzing]

With multi-page configuration like:

module.exports = {
  pages: {
    main: {
      entry: 'src/main.js',
      template: 'public/index.html',
      filename: 'index.html'
    },
    preview: {
      entry: 'src/preview.js',
      template: 'public/index.html',
      filename: 'preview.html'
    }
  }
}

HTML for page preview can only be accessed by localhost:8080/preview.html.

This plays well with vue-router + hash mode. But with history mode, preview.html would not be accessible.

eg. accessing localhost:8080/preview.html/about will result in index.html because /preview.html/about is a not found request and being fallback.

This PR adds support for multi-page fallback, it fallback ${baseUrl}/${name of page} to the correct html file (filename specified or ${name}.html).

In above case, the two pages could be accessed by localhost:8080/main and localhost:8080/preview.