getSetting: add an optional scope parameter

settings are stored in 3 places:
- login (.gpg file)
- store (.browserpass-settings.json)
- extension-level config (browser UI)

but we don't want each setting to be settable everywhere, so scope
allows one to control which of these 3 places to search for a given
setting.

src/helpers.js

@@ -32,10 +32,11 @@ module.exports = {
  * @param string key      Setting key
  * @param object login    Login object
  * @param object settings Settings object
+ * @param string scope    SettingScope value to specify where to search for the setting (optional)
  * @return object Setting value
  */
-function getSetting(key, login, settings) {
-    if (typeof login.settings[key] !== "undefined") {
+function getSetting(key, login, settings, scope = SettingScope.Login) {
+    if (scope === SettingScope.Login && typeof login.settings[key] !== "undefined") {
         return login.settings[key];
     }
     if (typeof settings.stores[login.store.id].settings[key] !== "undefined") {