Issue 12

Fixes #12

This is the simplest implementation I came up with. This is the first time for me to implement a command so I don’t know if I forgot something. Pleas take a look and let me know

Moreover I extracted the common specs for a command in a shared example. I don’t know if you like it or not, I thought that this will make it easy to add new commands. I intentionally avoid to test the help text because I think it’s duplication: you want to know if you forgot to write an help text for a command not the exact text, am I wrong? If you like it we could use it in all other command specs