title: How to add virtual property of type Array using Keystone.js?
body: <p>This is my code for my model: 'Info' and its the tokens property that creates the problem.</p>
  
  <pre><code>var keystone = require('keystone'),
      Types = keystone.Field.Types;
  var Info = new keystone.List('Info');
  Info.add({
      title: { type: String, required: true, initial: true },
      subtitle: { type: String, initial: true },
      content: { type: Types.Markdown, height: 500, initial: true },
      author: { type: Types.Relationship, ref: 'User', initial: true },
      date: { type: Types.Date, default: Date.now, initial: true },
      published: { type: Boolean, default: false, initial: true },
      tokens: { type: Array, virtual: true, noedit: true, collapse: true }
  });
  
  Info.defaultColumns = 'title, author, date|15%, published|15%'
  Info.register();
  </code></pre>
  
  <p>When running the app i get:</p>
  
  <pre><code>Error: Unrecognised field constructor: function Array() { [native code] }
  at List.field (/Users/bobmoff/Dropbox (Mobiento)/IMGNRY/Nordea/dev/code/rantebevis-cms/node_modules/keystone/lib/list.js:315:10)
  at List.&lt;anonymous&gt; (/Users/bobmoff/Dropbox (Mobiento)/IMGNRY/Nordea/dev/code/rantebevis-cms/node_modules/keystone/lib/list.js:200:16)
  at List.&lt;anonymous&gt; (/Users/bobmoff/Dropbox (Mobiento)/IMGNRY/Nordea/dev/code/rantebevis-cms/node_modules/keystone/lib/list.js:191:5)
  at List.&lt;anonymous&gt; (/Users/bobmoff/Dropbox (Mobiento)/IMGNRY/Nordea/dev/code/rantebevis-cms/node_modules/keystone/lib/list.js:230:5)
  at Function._.each._.forEach (/Users/bobmoff/Dropbox (Mobiento)/IMGNRY/Nordea/dev/code/rantebevis-cms/node_modules/keystone/node_modules/underscore/underscore.js:82:22)
  at List.add (/Users/bobmoff/Dropbox (Mobiento)/IMGNRY/Nordea/dev/code/rantebevis-cms/node_modules/keystone/lib/list.js:204:4)
  at Object.&lt;anonymous&gt; (/Users/bobmoff/Dropbox (Mobiento)/IMGNRY/Nordea/dev/code/rantebevis-cms/models/infos.js:4:6)
  at Module._compile (module.js:456:26)
  at Object.Module._extensions..js (module.js:474:10)
  at Module.load (module.js:356:32)
  </code></pre>