Add compression level and compression strategy attribute to pngoutput

As title, I add the attribute to adjust compression level and compress strategy.
For compression level, it is an integer between Z_NO_COMPRESSION(0) to Z_BEST_COMPRESSION(9).
For compression strategy, it one of the string in "default", "filtered", "huffman", "rle", or "fixed" that zlib support.
But I am confuse use which mode to be default mode, even there is a mode call default, but the zlib document says that rle is designed for png image.

From http://www.zlib.net/manual.html
"Z_RLE is designed to be almost as fast as Z_HUFFMAN_ONLY, but give better compression for PNG image data."