added a fullHeader wrapper

Hi,
I have added an option for add html at start of $container
example:

        $('#multiselect').multiSelect({
          fullHeader: "<div>your magic html!</div>"
        }

example of search in both lists :

    $('#multiselect').multiSelect({
      keepOrder: true,
      fullHeader: "<input type='text' id='multiselect_search' class='search-input' autocomplete='off' placeholder='Cerca'>",
      afterInit: function(ms){
        var that = this;
        var $search = $('#multiselect_search');
        selectableSearchString = '#'+that.$container.attr('id')+' .ms-elem-selectable:not(.ms-selected)',
        selectionSearchString = '#'+that.$container.attr('id')+' .ms-elem-selection.ms-selected';

        that.qs1 = $('#multiselect_search').quicksearch(selectableSearchString)
        .on('keydown', function(e){
          if (e.which === 40){
            that.$selectableUl.focus();
            return false;
          }
        });

        that.qs2 = $('#multiselect_search').quicksearch(selectionSearchString)
        .on('keydown', function(e){
          if (e.which == 40){
            that.$selectionUl.focus();
            return false;
          }
        });

      },
      afterSelect: function(){
        this.qs1.cache();
        this.qs2.cache();
      },
      afterDeselect: function(){
        this.qs1.cache();
        this.qs2.cache();
      }
    });