Add ESLint to package.json

This change adds ESLint as a dev-dependency and adds a lint script that
will run ESLint.

Author: MitMaro

package.json

@@ -4,7 +4,8 @@
   "description": "JSON Web Token implementation (symmetric and asymmetric)",
   "main": "index.js",
   "scripts": {
-    "test": "nyc --reporter=html --reporter=text mocha && nsp check && cost-of-modules"
+    "lint": "eslint .",
+    "test": "npm run lint && nyc --reporter=html --reporter=text mocha && nsp check && cost-of-modules"
   },
   "repository": {
     "type": "git",
@@ -34,6 +35,7 @@
     "chai": "^1.10.0",
     "conventional-changelog": "~1.1.0",
     "cost-of-modules": "^1.0.1",
+    "eslint": "^4.19.1",
     "mocha": "^2.1.0",
     "nsp": "^2.6.2",
     "nyc": "^11.8.0",