Enable no-unused-vars eslint rule

Without this rule enabled, it's too easy to accidentally leave in variables that you used to use but no longer need, which can make your code kind of confusing.

Author: maxjacobson

javascript/eslintrc.js

@@ -14,6 +14,7 @@ module.exports = {
  "eqeqeq": [2, "allow-null"],
  "no-shadow-restricted-names": 2,
  "no-undef": 2,
+ "no-unused-vars: 2,
  "no-use-before-define": 2,
  "quotes": [2, "double", "avoid-escape"],
  "radix": 2,