add Confirm prompt

[coryb]

Adding basic Y/n confirmation prompt. The implementation is similar to MultiChoice in that I have added an Answer that users can populate to get a bool result, otherwise it returns a Yes or No string depending on the answer. This will also loop if we get an answer that we cannot parse.

$ go run ./examples/confirm.go
? Are you happy? (Y/n) Yeah
✘ Sorry, your reply was invalid: "Yeah" is not a valid answer, try again
? Are you happy? Yes
response string: Yes
response happy: true

[AlecAivazis]

Hey Cory, thanks for yet another PR. I'll leave some comments inline.

I did some thinking about our problem today and what do you think about extending the analogy with json.Unmarhsal and changing the API of Ask to receive a pointer to a struct where the values for the entire survey is stored. We could use field tags to link up a question name with the field if we didn't want to enforce some conventions. This allows us to avoid the whole type problem to begin with from the consumer's perspective. It doesn't really affect this PR (since it makes 100% sense for Confirm to return a boolean), but I wanted to get your thoughts.

[AlecAivazis]

Pretty good overall! I think we should be a bit more defensive. Since the Answer field is not verified anywhere, it's easy to get a null-pointer reference if you fmt.Println(prompt.Answer)

[AlecAivazis]

What if the user doesn't provide an answer when instantiating this prompt?

[coryb]

doh! I meant to add that logic, forgot. Like in MultiChoice I just check to see if the value is nil and then create a default answer.

[AlecAivazis]

Awesome! Thanks for making those changes. I'm going to merge this in now.