Update throttle to use less naïve timing

See conversation in #6 

```
const tfn = throttle(100, fn)
tfn(); // fn();
// 99 ms later
tfn(); // setTimeout(fn, 100)
```

In this case the functions are executed 199ms apart while ideally you would want 100ms. I see that lodash/underscore versions of throttle are a little more intelligent about the delay provided to `setTimeout`. 