New: `mColumn` can now be given as a function to allow the column selection to be dynamically altered during run time.

* The function takes a single argument, the DataTables settings object,
  and is executed in the TableTools instance's scope.
* It returns an array of column indexes to be included in the output.

Example:

	$('#example').DataTable( {
		dom: 'T<"clear">lfrtip',
		tableTools: {
			aButtons: [ {
				sExtends: 'copy',
				mColumns: function ( ctx ) {
					var api = new $.fn.dataTable.Api( ctx );

					return api.columns('.selected').indexes().toArray();
				}
			} ]
		}
	} );