Terminal colors also for Prompt

[mdeknowis]

We would like to implement similar to existing bx plugins a colored prompt with variables like this:

But UI.Prompt(message string, options *PromptOptions) does not have a format input and args as e.g. Say(format string, args ...interface{}) has.

It would be an improvement, if we would be easily able to follow the bx style.

[weieigao]

@mdeknowis you can use fmt.Sprintf to format the message, and then call ui.Prompt.
The reason why UI.Prompt does not have format and args is args ...interface{} must be the last param, but I'd rather PromptOptions be the last since it's optional.

[mdeknowis]

That's a pity. Currently we try to avoid such mixtures and try to use only one stdio interface. Hence, we will not be behave like the other bx plugin. We hope, the IBM SDK will provide somthing in future.