Expose PNG output options requiring libimagequant #1484

lib/constructor.js

@@ -171,6 +171,10 @@ const Sharp = function (input, options) {
     pngProgressive: false,
     pngCompressionLevel: 9,
     pngAdaptiveFiltering: false,
+    pngPalette: false,
+    pngQuality: 100,
+    pngColours: 256,
+    pngDither: 1,
     webpQuality: 80,
     webpAlphaQuality: 100,
     webpLossless: false,