Support for Predicates

I need support for predicates. We are using `nested_config` as configuration for enabling/disabling features of the current fantasy platform:

```ruby
Features.configure do |feature|
  feature.galactic_users do |galagctic|
    galactic.enabled = true
    galactic.username_filter = /^hyper_.*/
  end
end

# ...

Features.config.galactic_users.enabled? # => true
Features.config.galactic_users.username_filter  # => /^hyper_.*/

Features.config.auto_commit.enabled? # => false
```