function appendUnique( list, value ) {
	if ( contains( list.length, list, 1, 0, value ) === false ) {
		list.push( value );
	}
	return list;
}
Appends a value to a list if that value is not already present in the list.

@private
@param {Array} list - list to which to append
@param {*} value - value to append
@returns {Array} input list