add generator comprehension transformer

lib/6to5/transformation/transformers/generator-comprehension.js

@@ -0,0 +1,15 @@
+var arrayComprehension = require("./array-comprehension");
+var t                  = require("../../types");
+
+exports.ComprehensionExpression = function (node, parent, file) {
+  if (!node.generator) return;
+
+  var body = [];
+  var container = t.functionExpression(null, [], t.blockStatement(body), true);
+
+  body.push(arrayComprehension._build(node, function () {
+    return t.expressionStatement(t.yieldExpression(node.body));
+  }));
+
+  return t.callExpression(container, []);
+};