func (o *Output) Printf(format string, a ...interface{}) {
	o.messages = append(o.messages, fmt.Sprintf(format, a...))
}
Printf formats a (success) message and adds it to the command output.
package cmd
import "fmt"
type Output struct {
	errors   []error
	messages []string
}
func (o *Output) Errorf(format string, a ...interface{}) {
	o.errors = append(o.errors, fmt.Errorf(format, a...))
}
func (o *Output) Error(a ...interface{}) {
	o.errors = append(o.errors, fmt.Errorf(fmt.Sprint(a...)))
}