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/tsconfig.json b/tsconfig.json
index d22d747..83adb1b 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -5,7 +5,10 @@
     "noImplicitReturns": true,
     "noFallthroughCasesInSwitch": true,
     "module": "es6",
-    "target": "es5",
+    "target": "es6",
+    "moduleResolution": "node",
+    "experimentalDecorators": true,
+    "useDefineForClassFields": false,
     "allowJs": true
   },
   "include": [