Overriding initialize

Is there a way to subclass `Hashr` and provide your own `initialize` method? Perhaps to do validation like so:

``` ruby
class Config < Hashr
  def initialize(options)
    super options
    raise ConfigError, "output is not defined"  unless output?
  end
end
```

...or for whatever other reason the user would want to subclass Hashr. This example doesn't work properly right now.
