Refactor options page to use Typescript

Also, I've added and ran eslint, and fixed several minor issues accross
the Typescript codebase.

Bug: translateselectedtext:15
Change-Id: I8cfd67697f9bfb22f6de93b64fd750de66bab863
diff --git a/src/common/consts.ts b/src/common/consts.ts
index 44e6ae8..9ec4dc4 100644
--- a/src/common/consts.ts
+++ b/src/common/consts.ts
@@ -1,10 +1,10 @@
-interface IsoLang {
+export interface IsoLang {
   name: string;
   nativeName: string;
-};
-interface IsoLangs {
+}
+export interface IsoLangs {
   [key: string]: IsoLang;
-};
+}
 
 export const isoLangs: IsoLangs = {
   'af': {'name': 'Afrikaans', 'nativeName': 'Afrikaans'},