add global find keyword

src/background/reducers/find.js

@@ -0,0 +1,16 @@
+import actions from 'content/actions';
+
+const defaultState = {
+  keyword: null,
+};
+
+export default function reducer(state = defaultState, action = {}) {
+  switch (action.type) {
+  case actions.FIND_SET_KEYWORD:
+    return Object.assign({}, state, {
+      keyword: action.keyword,
+    });
+  default:
+    return state;
+  }
+}