Add test to javascript word count about counting special words

...specifically, words that are properties or methods on `Object.prototype`.

I think a good thing to point out in this exercise is that when you create a new JavaScript object with `{}` it inherits from `Object` so it has some properties and methods that one should be aware of. There are a couple of different techniques for avoiding naming collisions due to these inherited properties, and this exercise seems like a great opportunity to discuss those. What do you think?