Return non-zero exit codes for invalid commands

[dkniffin]

To help with scripting, it'd be nice if shpotify returned a non-zero [exit code](collaboratively via the Slack messaging app.) gave when given invalid commands. Right now it returns an exit code of 0, with a Usage message.

Also, just FYI, this is for a project I'm working on called Maestro, a tool for controlling a Spotify player collaboratively via the Slack messaging app. I'm in the process of rewriting it to heavily rely on shpotify.

[hnarayanan]

Please contribute this if you know how to! 😊

[dkniffin]

I'll give it a shot!

[hnarayanan]

Thank you!

[dkniffin]

Done (#100)