accept optional function to use next

lib/middleware.js

@@ -17,13 +17,21 @@ module.exports.create = function (le) {
   }
 
   log(le.debug, "created middleware");
-  return function () {
+  return function (_app) {
+    if (_app && 'function' !== typeof _app) {
+      throw new Error("use le.middleware() or le.middleware(function (req, res) {})");
+    }
     var prefix = le.acmeChallengePrefix || '/.well-known/acme-challenge/';
 
     return function (req, res, next) {
       if (0 !== req.url.indexOf(prefix)) {
         log(le.debug, "no match, skipping middleware");
+        if (_app) {
+          _app(req, res, next);
+        }
+        else {
           next();
+        }
         return;
       }