Introduce deflateFactory option to packer

The main application motivating this addition is the creation of
reproducible compression results.  Since `zlib` is shipped with node.js, we
have no control over its version via npm.  To guarantee reproducible
results, other libraries (like pako) could be used, with a fixed version
number to guarantee consistent behavior.  This modification provides a hook
to allow such usage.

README.md

@@ -110,6 +110,7 @@ As input any color type is accepted (grayscale, rgb, palette, grayscale with alp
 - `deflateChunkSize` - chunk size used for deflating data chunks, this should be power of 2 and must not be less than 256 and more than 32*1024 (default: 32 kB)
 - `deflateLevel` - compression level for delate (default: 9)
 - `deflateStrategy` - compression strategy for delate (default: 3)
+- `deflateFactory` - deflate stream factory (default: `zlib.createDeflate`)
 - `filterType` - png filtering method for scanlines (default: -1 => auto, accepts array of numbers 0-4)
 
 

lib/packer.js

@@ -17,6 +17,7 @@ var Packer = module.exports = function(options) {
   options.deflateChunkSize = options.deflateChunkSize || 32 * 1024;
   options.deflateLevel = options.deflateLevel != null ? options.deflateLevel : 9;
   options.deflateStrategy = options.deflateStrategy != null ? options.deflateStrategy : 3;
+  options.deflateFactory = options.deflateFactory || zlib.createDeflate;
 
   this.readable = true;
 };
@@ -37,7 +38,7 @@ Packer.prototype.pack = function(data, width, height, gamma) {
   var filteredData = filter(data, width, height, this._options);
 
   // compress it
-  var deflate = zlib.createDeflate({
+  var deflate = this._options.deflateFactory({
     chunkSize: this._options.deflateChunkSize,
     level: this._options.deflateLevel,
     strategy: this._options.deflateStrategy