Allow pngquant's special exit code

https://github.com/pornel/pngquant/blob/6827a99fb9447abb6e2b58ab00f098f5106f8f88/README.md#--quality-min-max

> If conversion results in quality below the `min` quality the image won't be saved (if outputting to stdin, 24-bit original will be output) and pngquant will exit with status code 99.

(Fix #21)

index.js

@@ -75,7 +75,7 @@ module.exports = function (opts) {
 		});
 
 		cp.on('close', function (code) {
-			if (code) {
+			if (code && code !== 99) {
 				err = new Error(err);
 				err.fileName = file.path;
 				cb(err);