deep interpolation

Sometimes I need to get a key with a bunch of inner keys and I can't get them interpolated.

```yaml
people:
  ann: 'Ann is %{ann}'
  john: 'Ann is %{john}'
```

```ruby
I18n.t 'people', ann: 'good', john: 'big'
#=> { ann: 'Ann is %{ann}', john: 'John is %{john}' }

# Proposing this approach
I18n.t 'people', ann: 'good', john: 'big', deep_interpolation: true
#=> { ann: 'Ann is good', john: 'John is big' }
```