Allow add multiple entry points 1084

Created by: danvc

Hi reviewer.

The intention of this PR is to provide a solution for the following (3 years old) issue: https://github.com/facebook/create-react-app/issues/1084

The implementation doesn't require eject process.

The instruction to test it is:

1. Create your .html and .js file
2. Declare a new page in the property appPages in your package.json file. The structure must follow this shape:

    {
      "name": "login",
      "title": "login",
      "appHtml": "public/login.html",
      "appIndexJs": "src/login"
    },

3. Access it by changing the URL according the page. For example: `http://localhost:3000/login.html'

You can see it running on the following gif

Thank you so much by giving me this chance.