Allow custom log function. Not only console

lib/middleware.js

@@ -2,7 +2,7 @@
 
 var utils = require('./utils');
 
-function log(debug) {
+function _log(debug) {
   if (debug) {
     var args = Array.prototype.slice.call(arguments);
     args.shift();
@@ -15,6 +15,7 @@ module.exports.create = function (le) {
   if (!le.challenge || !le.challenge.get) {
     throw new Error("middleware requires challenge plugin with get method");
   }
+  var log = le.log || _log;
 
   log(le.debug, "created middleware");
   return function (_app) {