Added an example of the search API (#372)

* Added an example of the search API

* Fix manifest.json

menu-search/background.js

@@ -0,0 +1,26 @@
+/*
+Create a menu item for each installed search engine.
+The ID and title are both set to the search engine's name.
+*/
+function createMenuItem(engines) {
+  for (let engine of engines) {
+    browser.menus.create({
+      id: engine.name,
+      title: engine.name,
+      contexts: ["selection"]
+    });
+  }
+}
+
+browser.search.get().then(createMenuItem);
+
+/*
+Search using the search engine whose name matches the
+menu item's ID.
+*/
+browser.menus.onClicked.addListener((info, tab) => {
+  browser.search.search({
+    query: info.selectionText,
+    engine: info.menuItemId
+  });
+});