Added custom markup for placeholder as option

Added option for placeholder
- added custom markup for placeholder as option, useful when using with tables.
Example :
	// JS CODE
	$('table.data tbody').sortable(
	{  
		items: 'tr' ,
		placeholder : '<tr><td colspan="7">&nbsp;</td></tr>'
	}) ;

	// CSS STYLE FOR TR > TD
	.sortable-placeholder td
	{
		background:#FFFEB8;
		height:100px;
		width:100%;
		
	}

From https://github.com/bistoco/html5sortable/commit/f9c8e85b092217eaea5079cb34de2f56a10b525f