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.
No need to have this assertion.