Weighted choice

I think it would be useful to have a weighted version of `random.choice()`, that you could call like this:

```js
random.weightedChoice([
  ["oneSixth", 1],
  ["oneThird", 2],
  ["oneHalf", 3]
]);
```

And it'd add up all the weights, and for each item, its probability of being returned is the ratio of its weight over the total weight.

If you're ok with adding this, I'd be happy to send in a PR.