Fix use of core string dependency

.gitignore

@@ -6,3 +6,4 @@ sftp-config.json
 *.sublime-workspace
 .idea
 .vscode
+node_modules

library.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var S = require.main.require('string');
+var S = require('string');
 var meta = module.parent.require('./meta');
 var user = module.parent.require('./user');
 

package.json

@@ -37,5 +37,8 @@
   "license": "BSD-2-Clause",
   "bugs": {
     "url": "https://github.com/psychobunny/nodebb-theme-persona/issues"
+  },
+  "dependencies": {
+    "string": "^3.3.3"
   }
 }