feat: export the Root app in decap-cms-app, and add a flag to disable rendering

[eoneill]

It would be awesome if decap-cms-app exposed the Root component defined in decap-cms-core, and introduced a flag to disable rendering the app by default.

Is your feature request related to a problem? Please describe.
Somewhat. Decap doesn't always play nicely when it is inside of another React rendering context. Instead of using react-dom to directly render decap into a DOM node, it would be great to simply expose the component and let the hosting React app handle the rendering.

Describe the solution you'd like

import DecapCMS, { Root as CMSApp } from 'decap-cms-core';

function MyReactComponent() {
  DecapCMS.init({ config, render: false });
  DecapCMS.registerPreviewTemplate(...);
  return (
    <div>
      <CMSApp/>
    </div>
  );
}

Describe alternatives you've considered
With a deeper refactor, it would probably be good to disable the auto-init stuff as well by default, which would give more control to the app.

The final solution could be entirely component based...

import { Root as CMSApp } from 'decap-cms-core';

function MyReactComponent() {
  return (
    <>
      <CMSApp config={...}/>
    </>
  );
}