Add Omnibox API example (#164)

* Add Omnibox API example

* Add an example for the omnibox api

* Resolved pull request comments and did some minor refactoring

* Resolved pull request comments and did some minor refactoring

* Fix error when handling default suggestion

* Fix error when handling default suggestion

* Require Firefox 52, update version to 1.0, and update match pattern to be more specific

firefox-code-search/manifest.json

@@ -0,0 +1,18 @@
+{
+  "name": "Firefox Code Search",
+  "description" : "To use, type 'cs' plus a search term into the url bar.",
+  "version": "1.0",
+  "applications": {
+    "gecko": {
+      "strict_min_version": "52.0a1"
+    }
+  },
+  "background": {
+    "scripts": ["background.js"]
+  },
+  "omnibox": { "keyword" : "cs" },
+  "manifest_version": 2,
+  "permissions": [
+    "https://searchfox.org/"
+  ]
+}