Use unknown library.
  
    $ antigen-use unknown
    Usage: antigen-use <library-name>
    Where <library-name> is any one of the following:
     * oh-my-zsh
     * prezto
    [1]
  
  Missing argument.
  
    $ antigen-use
    Usage: antigen-use <library-name>
    Where <library-name> is any one of the following:
     * oh-my-zsh
     * prezto
    [1]
  
  Mock out the library loading functions.
  
    $ -antigen-use-oh-my-zsh () { echo Using oh-my-zsh. }
    $ -antigen-use-prezto () { echo Using prezto. }
  
  Note: We lack tests for these internal functions. I'm not sure how feasible
  testing them is given they most certainly use the network.
  
  Use oh-my-zsh library.
  
    $ antigen-use oh-my-zsh
    Using oh-my-zsh.
  
  Use prezto library.
  
    $ antigen-use prezto
    Using prezto.