I'm not really happy with the code duplication.

I thought about instead of making Uif an interface making it an wrapper around a Ui that exposes the Ui interface in addition to the formatter functions.

For example:

type Uif struct {
    Ui
}

func (u *Uif) Outputf(format string, v ...interface{}) {
     u.Output(fmt.Sprintf(format, v...))
}

Etc.

If that makes sense to you then I'm happy to re-visit the code. If you prefer the interface approach go ahead and merge this as is.

Please let me know if you have any change requests.