Add filter support for optional questions

Default value is now defined in metalsmith metadata when meta.js "when" property returns false. This makes the filter evaluation works for filters using data that have been skipped in the prompt workflow by using the default value of the field.

Close #408

Author: Toilal

lib/ask.js

@@ -32,18 +32,20 @@ module.exports = function ask (prompts, data, done) {
  */
 
 function prompt (data, key, prompt, done) {
-  // skip prompts whose when condition is not met
-  if (prompt.when && !evaluate(prompt.when, data)) {
-    return done()
-  }
-
   var promptDefault = prompt.default
   if (typeof prompt.default === 'function') {
     promptDefault = function () {
       return prompt.default.bind(this)(data)
     }
   }
 
+  // skip prompts whose when condition is not met
+  if (prompt.when && !evaluate(prompt.when, data)) {
+    // define default value to avoid failure on filter evaluations
+    data[key] = promptDefault;
+    return done()
+  }
+
   inquirer.prompt([{
     type: promptMapping[prompt.type] || prompt.type,
     name: key,