Add possibility to pass arbitrary vars from config.toml to script's env

At this moment, if you need to pass any extra variables to your script (i.e. apart from management user/pass and IP address to scan), you are on your own - you have to remember setting them every time you launch ralph-cli, which may be tedious and error-prone. I think it would be much more convenient to define them in `config.toml`, and let ralph-cli take care of passing them to your script's environment.

(FYI: @damian1976)