Return response in callback

Great plugin. I'm still pretty new to gulp so forgive me if this doesn't make any sense, but shouldn't the user input be returned to the callback? That way it would be possible to do something like this:

```
.pipe(prompt.checkbox('Select one:', ['a', 'b', 'c'], (val) ->

  switch val.value
    when 'a'
      type = 'patch'
    when 'b'
      type = 'minor'
    when 'c'
      type = 'major'

    ...
```