Not having j/k do up/down is confusing to me. Right now it's just [hardcoded](https://github.com/CITGuru/PyInquirer/blob/master/PyInquirer/prompts/checkbox.py#L205) to up/down keys. Either making j/k work by default, or maybe add something like `vi_style_movmenet=true` as an argument to `prompt` would make sense to me. Is this something that would be welcomed as a PR?
