zopflipng: add page (#2240)

pages/common/zopflipng.md

@@ -0,0 +1,11 @@
+# zopflipng
+
+> PNG image compression utility.
+
+- Optimize a PNG image:
+
+`zopflipng {{input.png}} {{output.png}}`
+
+- Optimize several PNG images and save with given prefix:
+
+`zopflipng --prefix={{prefix}} {{image1.png}} {{image2.png}} {{image3.png}}`