Features/support preselection and caption on options

1. Add ability to make an option pre-selected from `dataprovider()`.

  ```js
  var data = [
      { label: 'Cheese', value: 'cheese', selected: true },
      { label: 'Tomatoes', value: 'tomatoes' }
  ];
  $('multiselect').multiselect('dataprovider', data);
  ```

2. Add ability to set a title attribute on the option LABEL tags (visible on hover) from the `OPTION[title]` attribute.

  You can do it with the raw HTML select box:

  ```html
  <select id="example54" multiple="multiple">
    <option value="cheese" title="Smelly">Cheese</option>
    <option value="tomatoes" title="Handsome">Tomatoes</option>
    <option value="mozarella" title="Italian">Mozzarella</option>
  </select>
  ```

  Or you can do it with `dataprovider()` as follow:

  ```js
  var data = [
      { label: 'Cheese', value: 'cheese', caption: 'Swiss cheese' },
      { 
        title: 'This is an optgroup title',
        children: [
          { label: 'Cheese', value: 'cheese', caption: 'Smelly' },
          { label: 'Tomatoes', value: 'tomatoes', caption: 'Handsome' }
        ]
      }
  ];
  $('multiselect').multiselect('dataprovider', data);
  ```