pngcrush: add page

pages/common/pngcrush.md

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+# pngcrush
+
+> PNG image compression utility.
+
+- Compress a PNG file:
+
+`pngcrush {{in.png}} {{out.png}}`
+
+- Compress all PNGs and output to directory:
+
+`pngcrush -d {{path/to/output}} *.png`
+
+- Compress PNG file with best (but slowest) possible compression:
+
+`pngcrush -rem allb -brute -reduce {{in.png}} {{out.png}}`