Improvements to appsignal terminal command

[svanhesteren]

Couple of things that I think can be improved in the appsignal command line tool.

• When showing the help of it I miss an explanation of the commands.

$ appsignal --help   
                      
Usage: appsignal <command> [options]
    -v, --version                    Print version and exit
    -h, --help                       Show help and exit

Available commands: demo, diagnose, install

A bit of info on what the commands do would be very nice.

• Next when the push api key is wrong or missing the diagnose command tells you, but the demo command doesn't which can be confusing.