Custom HTML, JS, CSS

[gak]

Allow users to specify locations of custom assets that they host themselves. e.g. in config.json:

{
  "extraJS": [
    "customScript.js",
    "googleAnalytics.js"
  ],
  "extraCSS": [
     "https://fonts.googleapis.com/icon?family=Material+Icons"
  ],
  "introHTML": "./intro.html",
  "errorHTML": "./error.html"
}

introHTML would replace the introduction area with your own company specific intro. Similar with errorHTML, showing a custom error when a fetch request fails.

Also needs a documentation update.