Control of the WebView from an independent javascript instance.

# Feature Request

## Motivation Behind Feature

This feature would enable:
- Modifying html before loading it (i.e. rendering it with some templating engine).
- Smooth development using multi-page app architecture.

## Feature Description

An option to run Cordova apps with a "background" javascript instance that could control and communicate with the WebView.

## Alternatives or Workarounds

As an alternative you can use a single html file containing an iframe and scripts. Instead of loading desired pages in WebView the app would read the html file, render it adding mutation observers, write contents to a different file and then load that file in an iframe.

Drawbacks of the workaround:
- Apps become harder to debug.
- Unnecessary file reads/writes.
- Mutation observers have to be rendered into html in order to override link behavior. So that the iframe would send a message back to WebView making it read the next html file, render, write and load it in the iframe.
- On android platform `applicationDirectory` is read only, so you have to write the rendered html file to a different directory. In this case you have to either: copy all resources that are loaded using relative paths into the same directory or modify relative path when rendering html.