allow optional transformer to be set via the whitelist

lib/6to5/transformation/transformer-pass.js

@@ -29,7 +29,7 @@ TransformerPass.prototype.canRun = function () {
 
   // whitelist
   var whitelist = opts.whitelist;
-  if (whitelist.length && !contains(whitelist, key)) return false;
+  if (whitelist.length) return contains(whitelist, key);
 
   // optional
   if (transformer.optional && !contains(opts.optional, key)) return false;