`x` ... horizontal blur `y` ... vertical blur (optional)
function(args) {
                
                var x = _.isFinite(args.x) ? args.x : 2;

                return _.template('<filter><feGaussianBlur stdDeviation="${stdDeviation}"/></filter>', {
                    stdDeviation: _.isFinite(args.y) ? [x, args.y] : x
                });
            }