Usage:
figaro dokku:set --server=<example-dokku-server.com> --app=<example-app>

Limitations:
1. Requires Dokku server and app to be specified via --app and --server
2. Requires Dokku server to be accessible via ssh from the client machine for the 'dokku' user

Future enhancements could include supporting arbitrary users (which would require dynamically changing the command) and potentially reading the app and server values from the client/shell ENV

Also fix (as best as possible) minor command line help formatting issues