Add isMaster util function

utils/config.js

@@ -0,0 +1,20 @@
+'use strict';
+
+const config = require('../config');
+const eq = require('./eq');
+
+const masterById = /^\d+$/.test(config.master);
+const masterByUsername = /^@?\w+$/.test(config.master);
+
+if (!masterById && !masterByUsername) {
+	throw new Error('Invalid value for `master` in config file: ' +
+		config.master);
+}
+
+const isMaster = masterById
+	? user => user.id === Number(config.master)
+	: user => user.username && eq.username(user.username, config.master);
+
+module.exports = {
+	isMaster,
+};