Add support for string plurals

A way to deal with string plurals would be useful.

```
{
  "clicked": {
    "zero": "You didn't click",
    "one": "You clicked one time!",
    "other": "You clicked many times!"
  }
}
```

In code the strings would be accessed as...

```
Translation.of(context).clicked(times);
```
Where 'times' is an integer that would be used to map to plural entry.
