If you give a value for "unused" that's not in `warnable_types` and you have an unused variable, you get the following error:

```
/Users/tupton/code/jshint/src/stable/jshint.js:3581
				throw err;
				      ^
TypeError: Cannot call method 'indexOf' of undefined
    at warnUnused (/Users/tupton/code/jshint/src/stable/jshint.js:3486:50)
    at nt (/Users/tupton/code/jshint/src/stable/jshint.js:3558:7)
    at Array.forEach (native)
    at itself (/Users/tupton/code/jshint/src/stable/jshint.js:3529:14)
    at lint (/Users/tupton/code/jshint/src/cli/cli.js:272:7)
    at exports.run.verbose (/Users/tupton/code/jshint/src/cli/cli.js:315:4)
    at Array.forEach (native)
    at Object.exports.run (/Users/tupton/code/jshint/src/cli/cli.js:314:9)
    at Object.exports.interpret (/Users/tupton/code/jshint/src/cli/cli.js:372:24)
    at Object.<anonymous> (/Users/tupton/code/jshint/bin/jshint:3:32)
```

Checking for `unused_opt in warnable_types` or just `warnable_types[unused_opt]` before checking `indexOf` ought to do the trick.