Add `style` option

This adds a `style` option with the possible values: `"best fit"` (default), and `"numeric"`.

When the `{style: "numeric"}` is set, the formatted output will _always_ contain a number. Whereas, when the default `{style: "best fit"}` is used, relative units (e.g., "yesterday") might be chosen as the best output for the given locale and relative date.

Tests have also been added for both `options`.

----

```js
var yesterday = Date.now() - (20 * 60 * 60 * 1000);
var bestFit   = new IntlRelativeFormat('en');
var numeric   = new IntlRelativeFormat('en', {style: 'numeric'});

console.log(bestFit.format(yesterday)); // => "yesterday"
console.log(numeric.format(yesterday)); // => "1 day ago"
```