Enable embedding other new tab chrome extensions

[JRJurman]

It would be really awesome if we could have other new tab pages live as a card in this chrome extension!

This could be useful if people already have new tab chrome extensions that they enjoy using - such as Tabby Cat or Tab For A Cause.

It seems that it's possible to point to an extension id - alternatively we may be able to read other chrome extensions installed and get the id from there - the difficult part is injecting that into new-group-nav 🤔

I've tried doing that with an iframe, which seems to work for web-pages, but doesn't work for other chrome extensions: https://github.com/JRJurman/new-group-nav/compare/iframe-chrome-extension