user-show: add --out option to save certificates to file
    
    Add the --out option to user-show, bringing it into line with
    host-show and service-show with the ability to save the user's
    certificate(s) to a file.
    
    https://fedorahosted.org/freeipa/ticket/5171
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>