Add new category concurrency

Some problems is in category concurrency, missing concurrency category will make some problems disappear in list commands.
You can try problem 1114, which is missing without concurrency.

Author: xpunch

lib/config.js

@@ -10,7 +10,8 @@ const DEFAULT_CONFIG = {
     categories: [
       'algorithms',
       'database',
-      'shell'
+      'shell',
+      'concurrency'
     ],
     langs: [
       'bash',