add feedback link.

app/config.js

@@ -9,4 +9,5 @@ module.exports = {
     VERSION: '3.0.0'
     , url_chk_version: 'http://oldj.github.io/SwitchHosts/v.txt'
     , url_homepage: 'http://oldj.github.io/SwitchHosts/'
+    , url_feedback: 'https://github.com/oldj/SwitchHosts/issues'
 };

app/main.js

@@ -174,6 +174,11 @@ app.on('ready', function () {
             click: function () {
                 require('electron').shell.openExternal(config.url_homepage);
             }
+        }, {
+            label: 'Feedback',
+            click: function () {
+                require('electron').shell.openExternal(config.url_feedback);
+            }
         }]
     }];