Introduce `backup` plugin to export/import settings

This PR introduces two new commands to export and import your dokku settings. The idea is that you can generate a tar with all your settings (app env, vhost etc) and use it for example to easily re-setup dokku after you re-install your host.

You can either backup to a file with `dokku backup:export <file>` or get a tar stream on stdout `dokku backup:export > myfile.tar`. The same but then with stdin for `dokku backup:import`.

The backup file is versioned with a file in the root of the backup (currently v1). All calls to hooks also include this version number. This way we can hopefully evolve the backup format without breaking setups.