Add simple `--help` or `-h` for skm

[alexcu]

It would be useful to know what commands are available to SKM (i.e., go through each command available and provide a description value) without having to reference SKM (unless I am missing some documentation here?). I believe there is something already like this in the args parser...

[jakerenzella]

@alexcu this work has been started with the help.js file