separate setting actions and reducers

src/background/reducers/setting.js

@@ -0,0 +1,17 @@
+import actions from 'settings/actions';
+
+const defaultState = {
+  value: {},
+};
+
+export default function reducer(state = defaultState, action = {}) {
+  switch (action.type) {
+  case actions.SETTING_SET_SETTINGS:
+    return {
+      value: action.value,
+    };
+  default:
+    return state;
+  }
+}
+