fix(listeners): fix listeners typo

Closes #290

dist/showdown.js

@@ -836,7 +836,7 @@ showdown.Converter = function (converterOptions) {
           outputModifiers.push(ext[i]);
           break;
       }
-      if (ext[i].hasOwnProperty(listeners)) {
+      if (ext[i].hasOwnProperty('listeners')) {
         for (var ln in ext[i].listeners) {
           if (ext[i].listeners.hasOwnProperty(ln)) {
             listen(ln, ext[i].listeners[ln]);

src/converter.js

@@ -126,7 +126,7 @@ showdown.Converter = function (converterOptions) {
           outputModifiers.push(ext[i]);
           break;
       }
-      if (ext[i].hasOwnProperty(listeners)) {
+      if (ext[i].hasOwnProperty('listeners')) {
         for (var ln in ext[i].listeners) {
           if (ext[i].listeners.hasOwnProperty(ln)) {
             listen(ln, ext[i].listeners[ln]);