move currency to config file; change units to their own group in settings

resources/stubs/installer/config.stub

@@ -21,6 +21,15 @@ return [
     'phpvms' => [
         'skin' => 'default',
         'vacentral_api_key' => '',
+
+        /**
+         * The ISO "Currency Code" to use
+         * http://www.xe.com/iso4217.php
+         *
+         * Note, do not change this after you've set it, unless you don't
+         * care that the currencies aren't "exchanged" into the new format
+         */
+        'currency' => 'USD',
     ],
 
     # overrides cache.php