Added support to display optgroup name as prefix for value in multiple instances

Added optgroup labels to options to help with clarity when select has multiple enabled.

Example:

    <select class="prefix" multiple="multiple">
        <optgroup label="Group1">
            <option>itemA</option>
            <option>itemC</option>
        </optgroup>
        <optgroup label="Group2">
            <option>itemA</option>
            <option>itemB</option>
            <option>itemD</option>
        </optgroup>
    </select>
    $('.prefix').select2({groupPrefix: true });

Displayed selected items will now look like:

<kbd>Group1: itemA</kbd> <kbd>Group2: itemA</kbd> <kbd>Group2: itemB</kbd>