Add support for imagemagick stacks

Fixes #204.

README.md

@@ -45,7 +45,8 @@ gm('/path/to/my/img.jpg')
 
 ```js
 var fs = require('fs')
-  , gm = require('gm');
+  , gm = require('gm')
+  , im = require('gm').subClass({ imageMagick: true });
 
 // resize and remove EXIF profile data
 gm('/path/to/my/img.jpg')
@@ -126,6 +127,24 @@ gm(200, 400, "#ddff99f3")
 .write("/path/to/brandNewImg.jpg", function (err) {
   // ...
 });
+
+// also supports imagemagick stacks
+im('test.psd[0]')
+.rotate('green', '-90') // rotates only once for both outputs
+.pipeline()
+  .clone(0)
+  .autoOrient()
+  .resize(100, 100)
+  .write('test100.png')
+.close()
+.pipeline()
+  .clone(0)
+  .resize(500, 500)
+  .write('test500.jpg')
+.close()
+.write("NULL:", function (err) { // throw away the collection of layers
+  // ...
+});
 ```
 
 ## Streams

index.js

@@ -118,6 +118,7 @@ require("./lib/command")(gm.prototype);
 require("./lib/compare")(gm.prototype);
 require("./lib/composite")(gm.prototype);
 require("./lib/montage")(gm.prototype);
+require("./lib/pipeline")(gm.prototype);
 
 /**
  * Expose.

lib/pipeline.js

@@ -0,0 +1,39 @@
+
+module.exports = function (proto) {
+  proto.pipeline = function () {
+    if (!this._options.imageMagick) {
+      throw new Error('Batching is not supported by GraphicsMagick');
+    }
+
+    var gm = new IMPipeline(this, arguments);
+    gm.options(this._options);
+    return gm;
+  };
+
+  function IMPipeline(parent, args) {
+    proto.constructor.apply(this, args);
+    this._pipeline = parent;
+  }
+  IMPipeline.prototype = Object.create(proto);
+
+  IMPipeline.prototype.write = function (name) {
+    this.out('-write', name);
+    return this;
+  }
+
+  IMPipeline.prototype.clone = function (idx) {
+    if (idx !== undefined) {
+      this.in('-clone', idx.toString());
+    } else {
+      this.in('+clone');
+    }
+    return this;
+  };
+
+  IMPipeline.prototype.close = function () {
+    this._pipeline.out('(');
+    this._pipeline.out.apply(this._pipeline, this.args().slice(1, -1));
+    this._pipeline.out(')');
+    return this._pipeline;
+  };
+};