Add translatePlural filter

Issue #104 

This adds `translatePlural` filter with syntax
```
{{'One rock' | translatePlural:rocks.length:'{} rocks'}}
```

This is a bit different with what @rubenv suggested
```
{{"1 car" | translatePlural:n:"$count cars"}}
{{"1 car" | translatePlural:n:"$count cars":"domain"}} // With a domain
```
1. Use `{}` instead of `$count`. That's because the probability of collision of `$count` is higher than `{}` I think. Matching `{}` is also much easier.
2. Domain is not supported as I'm not too familiar with it. It can be added later on if somebody needs it.
This should become:

```js
return gettextCatalog.getPlural(n, string, stringPlural, {
    $count: n,
});
```

That should make the `{{$count}}` placeholder work.