Make prompts when support function.

[likun7981]

Now we only set prompts when is an attribute from the other prompts , and the prompt must be a boolean . but sometimes we want set the when to a special value, when a prompt choice the value , we return a boolean.
so I resolve this problem; I through this example to illustrate;

previously , we can only set prompt like this

q1: {
  type: 'confirm',
  require: true,
  message: 'this is a question?',
  default: true
},
q2: {
  when: 'q1',
  type: 'list',
  message: 'Which choice ?',
  choices:[] // some choices
}

If q1 is not a confirm , it is a list, like this

q1: {
  type: 'list',
  message: 'make choice?',
  choices:['c1','c2'] // some choices
},
q2: {
  when: ????,
  type: 'list',
  message: 'Which choice ?',
  choices:[] // some choices
}

now we want to when q1 choice c1 , then ask q2, otherwise do not ask q2;

so when should like inquirer to support set function value to when

we can set like this:

q1: {
  type: 'list',
  message: 'make choice?',
  choices:['c1','c2'] // some choices
},
q2: {
  when: function(answers){
    return answers.q1 === 'c1';
  },
  type: 'list',
  message: 'Which choice ?',
  choices:[] // some choices
}

[posva]

Can you add a test for this, please? 🙂

[likun7981]

The test is ok

[posva]

ping @egoist @zigomir

[zigomir]

Looks good to me! I only maybe miss a test case that exercises previous when functionality. I know this is not a concern for this PR, but it might be useful to prevent any regressions.

[yyx990803]

Thanks for the PR and sorry for letting it sit for so long. As of now, we are no longer adding new features of vue-cli 2.x, so I am closing this PR. Again, thanks for the contribution.