add end/start user transformers

lib/6to5/file.js

@@ -192,13 +192,25 @@ File.prototype.transform = function (ast) {
 
   var self = this;
 
-  _.each(this.opts.transformers, function (transformer) {
-    transformer.transform(self);
-  });
+  var runUserTransformers = function (end) {
+    _.each(self.opts.transformers, function (transformer) {
+      if (transformer.opts.end) {
+        if (!end) return;
+      } else if (end) {
+        return;
+      }
+
+      transformer.transform(self);
+    });
+  };
+
+  runUserTransformers();
 
   _.each(transform.transformers, function (transformer) {
     transformer.transform(self);
   });
+
+  runUserTransformers(true);
 };
 
 File.prototype.generate = function () {

lib/6to5/transformation/transformer.js

@@ -4,8 +4,9 @@ var traverse = require("../traverse");
 var t        = require("../types");
 var _        = require("lodash");
 
-function Transformer(key, transformer) {
+function Transformer(key, transformer, opts) {
   this.transformer = Transformer.normalise(transformer);
+  this.opts        = opts || {};
   this.key         = key;
 }