Add "_subCommand" param.

Use to select the graphicsMagick sub-command needed.

index.js

@@ -28,6 +28,7 @@ function gm (source, height, color) {
   this._in = [];
   this._out = [];
   this._outputFormat = null;
+  this._subCommand = 'convert';
 
   if (source instanceof Stream) {
     this.sourceStream = source;

lib/args.js

@@ -11,7 +11,13 @@ var argsToArray = require('./utils').argsToArray;
  */
 
 module.exports = function (proto) {
-
+  
+  // define the sub-command to use, http://www.graphicsmagick.org/utilities.html
+  proto.subCommand = function subCommand (name){
+    this._subCommand = name;
+    return this;
+  }
+  
   // http://www.graphicsmagick.org/GraphicsMagick.html#details-adjoin
   proto.adjoin = function adjoin () {
     return this.out("-adjoin");

lib/command.js

@@ -239,7 +239,7 @@ module.exports = function (proto) {
   	if (this._outputFormat) outname = this._outputFormat + ':' + outname;
 
     return [].concat(
-       'convert'
+        this._subCommand
       , this._in
       , source
       , this._out