Enable the user to add and display descriptions to commands.

[fpgmaas]

Make the following possible:

test:
  my-command:
    cmd: "echo Hello! My name is: $name. My favourite fruit is: $fruit"
    description: Print someone's name and their favourite fruit.
    echo: false
    args:
      - name
      - fruit: apple

Also allow the user to view these descriptions in the terminal when running ckit. Displaying all descriptions would require quite a large overhaul of the Picker and GrouPicker classes. Maybe a easier way to get started would be to display the description when the user presses 'h' when the cursor is focussed on an option.