Couple of Fixes

[anta-semenov]

1. Fix for new yeoman-generator prompt api. In new api prompt should return a promise. So there were an error in Atom: “Can’t find .then of undefined” Now if callback doesn’t provide prompt method of atom-adapter will return promise

2.Add ability for "when" property of questions. When description from Inquirer.js: when: (Function, Boolean) Receive the current user answers hash and should return true or false depending on whether or not this question should be asked. The value can also be a simple boolean.