Feature: Enable export of indices, settings and rules to backup an app

It's possible to export indices, setting and rules one by one using the CLI and piping to files, but it would be handy to be able to export all indices, settings and rules for an application and save them to files to enable backing up of an application.

This would enable customers to backup and restore / import applications.

The [old cli](https://github.com/algolia/algolia-cli-old) had the option to `export` indices and settings to a file.

I've created a Gist with a script using the new cli that can be used as a workaround to backup an application: https://gist.github.com/keybits/ce86d66cc02dcfe063531945ba8a3f9b (no restore / import script yet)