add global find keyword

test/background/reducers/find.test.js

@@ -0,0 +1,19 @@
+import { expect } from "chai";
+import actions from 'background/actions';
+import findReducer from 'background/reducers/find';
+
+describe("find reducer", () => {
+  it('return the initial state', () => {
+    let state = findReducer(undefined, {});
+    expect(state).to.have.deep.property('keyword', null);
+  });
+
+  it('return next state for FIND_SET_KEYWORD', () => {
+    let action = {
+      type: actions.FIND_SET_KEYWORD,
+      keyword: 'cherry',
+    };
+    let state = findReducer(undefined, action);
+    expect(state).to.have.deep.property('keyword', 'cherry')
+  });
+});