Add support for ImageMagick specfic crop options

Support ImageMagick crop function specific options ! and @.

lib/args.js

@@ -789,7 +789,7 @@ module.exports = function (proto) {
   }
 
   // http://www.graphicsmagick.org/GraphicsMagick.html#details-crop
-  proto.crop = function crop (w, h, x, y, percent) {
+  proto.crop = function crop (w, h, x, y, arg) {
     if (this.inputIs('jpg')) {
       // avoid error "geometry does not contain image (unable to crop image)" - gh-17
       var index = this._in.indexOf('-size');
@@ -798,7 +798,10 @@ module.exports = function (proto) {
       }
     }
 
-    return this.out("-crop", w + "x" + h + "+" + (x || 0) + "+" + (y || 0) + (percent ? '%' : ''));
+    // backward compatibility 
+    arg = (arg === true) ? '%' : arg;
+
+    return this.out("-crop", w + "x" + h + "+" + (x || 0) + "+" + (y || 0) + (arg || ''));
   }
 
   // http://www.graphicsmagick.org/GraphicsMagick.html#details-magnify

test/crop.js

@@ -17,6 +17,13 @@ module.exports = function (gm, dir, finish, GM) {
   var args2 = m2.args();
   assert.equal('200x155+300+0%', args2[3]);
 
+  var m3 = GM(dir + '/image.png')
+  .crop(200, 155, 300, 0, '!');
+
+  var args3 = m3.args();
+  assert.equal('200x155+300+0!', args3[3]);
+
+
   if (!GM.integration)
     return finish();