Run a `search` function in vi mode for List's

When `vi` mode is enabled on a `List` and the user presses `/` then run a `search` function that set as a property on the options object of the List. 

This will then do a _regexp search_ based on what value the `callback` passed  to `search` is called with. e.g: 

```javascript
var countries = blessed.list({
  width: '30%',
  height: '95%',
  scrollable: true,
  border: {
    type: 'line'
  },
  keys: true,
  vi: true,
  search: function(cb){
     return cb('turkey')
  }
});

countries.setItems(['australia', 'italy', 'turkey'])
```