Expand prompt logic for PR #199

[bartdeboer]

The Prompt function from PR #199 could be a bit expanded to support message formatting and default values:

// Prompt creates a pipe that reads user input from stdin after displaying the
// specified prompt.
func Prompt(prompt string) *Pipe {
	return Promptf(prompt)
}

// Promptf displays a formatted prompt to the user and reads a line of input.
// Additional arguments can be provided for the format specifier like Printf. 
// The last argument will be used as the default value.
func Promptf(format string, args ...any) *Pipe {
	fmt.Printf(format, args...)
	p := Stdin().First(1)
	input, err := p.String()
	if err != nil {
		return p
	}
	input = strings.TrimSpace(input)
	if input == "" && len(args) > 0 {
		input = fmt.Sprint(args[len(args)-1])
	}
	return Echo(input)
}

Example:

input, _ := script.Prompt("Input: ").String()
fmt.Printf("Input was %s\n", input)

input, _ = script.Promptf("Input [%s]: ", "John").String()
fmt.Printf("Input was %s\n", input)

Output:

Input: Jane
Input was Jane

Input [John]:
Input was John