add canCompile method to node api

lib/6to5/index.js

@@ -1,4 +1,5 @@
 var transform = require("./transformation/transform");
+var path      = require("path");
 var fs        = require("fs");
 var _         = require("lodash");
 
@@ -14,6 +15,12 @@ exports.polyfill = function () {
   require("./polyfill");
 };
 
+exports.canCompile = function (filename, altExts) {
+  var exts = altExts || [".js", ".jsx", ".es6"];
+  var ext = path.extname(filename);
+  return _.contains(exts, ext);
+};
+
 exports.transform = transform;
 
 exports.transformFile = function (filename, opts, callback) {