Allow keys weight.

Fuse does supports specifying the keys as an array of objects like the following, would be super nice to have. It permits to sort the results depending on the weight the field that matched has. 😋 

```js
const KEYS_TO_FILTER = [
  { name: 'item.title', weight: 1.0 },
  { name: 'item.description', weight: 0.8 },
  { name: 'item.author', weight: 0.4 }
];
```