Add eslint check to npm run test

Currently, CI build runs npm run test, which doesn't include eslint check.

One approach would be -- in package.json, add npm run lint to pretest?

{ ....,
  "scripts": {
    "lint": "eslint index.js src",
     ...
     "pretest": "npm run lint && npm run build && browserify -s tape -r tape -o test/tape.js",
  },
...
}