[jshintrc] Don't warn when variables are defined but never used

This pull request changes the JSHint config to *not* warn when defined variables are not used.

Currently, if you don't use an argument in a function, you'll get a warning. I think this will result in a lot of unnecessary warnings. A common example is route handlers and middleware.

```js
function(req, res, next) {
  res.send(200);
}
```

This results in two warnings; one for `req` and one for `next`.

I think it's useful to keep these arguments. Even though an argument may not be used at this time, in the future it may be, and for people not as familiar with certain functions, it would be beneficial to have them already listed.